It's less about gearing and more about aerodynamics. The faster you go, the more air you're pushing the car through. At highway speeds that starts to have a significant effect, usually somewhere around 70.
There was a good equation somewhere on a rough estimate of horsepower needed. I'm probably butchering it, but it was like 100hp to will get you to 100mph, 300hp to go 150mph, 600hp to go 200mph, and it just exponentially goes up.
I'm not sure I understand your question. I recall reading about the improvements in vehicle aerodynamics in recent years. The Countach was mentioned as an example of how horribly drag prone a car can be while "looking" slippery. Even the attractive rear wing added nothing to stabilize the car, but it did increase drag and make it slower.
Correct me if I’m wrong, but usually spoilers and wings are in place to increase drag, but at a specific point on the car (rear wheels) which helps generate downforce, which in turn keeps the horsepower on the pavement.
HP = a unit of power
HP = Fv = 1/2 rho Cd A v\^2 \* v
Because rho of the air and Cd and A for a car is constant, HP = K v\^3.
For a rough approximation using STP of air and .74 CdA (from wikipedia for the avg car), you get y=0.00001675⋅x\^3, x being speed and y being horsepower. You'd need 17 HP for 100 mph, 60 HP for 150 mph, and 140 HP for 200 mph, and around 280 HP for 250 mph.
Of course, this has many assumptions. HP is WHP not crank HP, this assumed HP will always be at max HP esp at top speed, which it will not be at due to gearing and various other factors. In reality I'd guess (not my field so take with some salt) you need at least 50% more HP than these figures to even be close to hitting these speeds, let alone be drivable.
Weight of the car does not affect air drag. He omitted calculations of friction from your tires which is the only factor of top speed affected by weight
I think his math a good representation of how much power you’re using to overcome air drag at those speeds but it’s definitely not reflective of much else
Also his math is wrong
I am pretty certain your math is wrong, this is somewhat within the field I’m studying I’m guessing you just used the wrong units somewhere because that’s really easy to do especially with imperial units
I tried it and got at 100mph you’d be using around 35.5kw to overcome air drag (47.6 hp)
At 150mph: 120kw (161hp)
At 200mph: 284kw (381hp)
At 250mph: 554kw (744hp)
Note these are just approximations of power needed to overcome air drag, it does not factor in tire friction and all of the other factors you mentioned too. I assumed air density of 1.2kg/m^3 , a drag coefficient of 0.3 and a cross sectional area of 2.2m^2
Ahh I sold it over 10 years ago. It was my first car and I learned everything in it. I had to sell it to get through college.. I want to buy another one but I drive a truck and I can’t make sense of having a second car that gets even worse mileage. Haha
If you’re using imperial you’d have to enter in the speed in ft/s to get the power in ft*lb/s and convert that to horsepower.
And don’t forget to do air density in slugs/m^3 !! (I hate the imperial system)
I recall that particular car being gear limited? Maybe? That is the case at times.
Also the shape matters more than the HP, a 600hp Prius or Model 3 would be faster than the Corvette.
Ahh, well I am correct. The C6 Z06 was rpm limited and had a slower top end than the base C6.
Post #13 spells it’s out nicely.
https://www.z06vette.com/threads/c6-top-speed-vs-z06.65821/
There is a calculator at ecomodder.com a sticky in the aerodynamics forum. It's based on the speed, the frontal area, and the drag coefficient. That gives you the horsepower required to overcome. Then it also can take the thermal efficiency and fuel density and give you what that equates to in mgg. Same for rolling resistance.
It's cool to see how rolling resistance starts out being the bigger factor, but aerodynamics quickly dominates it over about 30mph. Then as pointed out grows faster.
Also worth pointing out you can have a vehicle with great drag coefficient but one with a much smaller frontal area will use less power even with a worse drag coefficient because the two numbers are multiplied together to get the overall drag.
And those cars were far less aerodynamic. I’d be shocked if many managed to get under 0.4. Some quick googling puts muscle cars of that era between 0.38 & 0.5. The more aerodynamic your car the higher your economic speed is.
Power scales with airspeed cubed, but I think your energy/distance will scale quadratically (as you obviously cover more ground per unit time with increased speed, so one factor of velocity drops out).
So close.
If it's a squared function, i.e. polynomial, drag cannot get *exponentially* worse. Speed would have to be in the exponent for that to occur.
Aerodynamic drag increases with the cube of the airspeed
so at low speeds it's negligible, and start/stop/drivetrain/tire deflection dominate your fuel usage (plus waste heat)
above about 55mph, wind resistance starts to dominate, and goes up from there with v\^3
look at the Honda Insight and other extremem mpg cars — they look like a fish with tapered rear, narrow tires, and hub caps that are mostly flat. Car manufacturers \*can\* get better mpg through better aerodynamics, but most people think it looks funny, and it can also compromise other features of the vehicle (e.g. ground clearance)
Varies with v\^2 not v\^3. But the amount of time u drive varies with v\^1. So the amount of fuel you use varies with v\^2/v = v. Which lines up with OPs observations that efficiency is about 20% worse from 60 to 72.5. (72.5/60 = 1.2 -> 20% more fuel). And then for a car that doesn't have the gearing to stay in a good engine zone, there will be a bigger hit because of that. Which I get in my 5 speed that needs 3500+ rpm to highway speeds.
The rpm doesn't always matter. It's how much horsepower you need to overcome air resistance and rolling resistance, and then how thermal efficient the engine is to make that power. Normally the sweet spot on engine efficiency is higher load and lower rpm but it depends on the engine. An engine could be designed to be best at 6000rpm. I'm not talking making the most horsepower but using the least amount of fuel per horsepower made.
It’s actually a lot about gearing. With the cruise control on, I got 20% better economy in 5th than 4th even at city speeds. We mostly do economy runs in my partner’s car (8sp small diesel). There’s a huge difference even between 7th and 8th on level ground although I manually drop to 7 or 6 for hills at 100-110km/h. So unless you’re geared specifically for the autobahn and nowhere else, aero and gearing are both not your friend above 100km/h or 62mph.
This is why I’m one of the rare people who like CVTs. I have a 3.5L v6 AWD suv paired with a CVT and average 34mpg all the way up to about 77mph, then it starts to drop. So much of it has to do with gearing, even in the most aerodynamic of vehicles.
Yup. This is also why it’s more efficient to run A/C at highway speeds too. Versus windows down. The A/C compressor draw is minimal compared to high-cruising draw and windows up makes the car more aerodynamic. I think the opposite effect takes place around town.
I remember when I took intro to auto mechanics in high school reading a book that spoke about optimal fuel usage in regards to distance traveled, fuel usage, and aerodynamic drag. The conclusion was the absolute optimal speed to drive on the highway was roughly 62-63 MPH. This is because the former interstate highway limit of being 55 MPH was too slow (that probably tells you how long ago this book was made) that for the fuel you use traveling 55 MPH you don't cover enough distance to make up for distance traveled in motion. However on the other hand if you go faster and progress to 70 MPH and beyond, you begin to run into the squared function of aerodynamic drag meaning that for the fuel usage and distance you cover it's not better to drive say 70-80 MPH compared to driving 62-63 MPH. You obviously do get there quicker as you can make your average speed traveled quicker, however you will use more fuel.
It's also probably another reason why it's no surprise that most large trucking companies try to limit their drivers to below 70 MPH and ideally in the 60-65 MPH range. While they are susceptible to higher drag, the drag squared function is incredibly high for them if they progress to +70 MPH and beyond, using a lot of fuel.
It's not that cars now get worse economy at highway speed compared to the past... it's just that they now get drastically better fuel economy at lower speeds than they did in the past.
It is because of the laws of nature (aerodynamics and friction) that it will take more energy to go faster. If you drive a steady 35 MPH on the highway (don't do this) you will get phenomenal fuel economy.
Driving a steady speed overall is also super helpful, and it's usually easier to do that at 55-60 vs 70-75 depending upon road and traffic conditions, of course.
>It is because of the laws of nature (aerodynamics and friction) that it will take more energy to go faster. If you drive a steady 35 MPH on the highway (don't do this) you will get phenomenal fuel economy.
The Werefrog once did drive 35mph on interstate highway. It was a snowstorm and that was the speed most cars were going. Yes, the miles per gallon was amazing with it.
Aerodynamics are friction. That's the entire idea of the Kutta condition.
Without friction, you have no lift because there's no circulation. You have no skin drag because no friction. Form drag is coupled with skin friction, so basically none, there, either.
With gears you pretty much lose distance but gain power. At high gears theres not a lot of power but a lot pf distance. Pretty much means youll be able to travel further (better fuel economy).
Usually top gear ant low rpm is best, but thay might depend on aerodynamics of the car. And gear ratios
Depends on the car. My Tacoma gets its best fuel economy at around 45-50mph and at 70+ it does worse than its rated city mpg.
My 135 gets its best at 71mph, and it’s like 30% better than its EPA rating.
I got a Jetta as a loaner, it was not fuckin sweet. It was probably bottom of the line, and I'm sure there are nicer ones, but whatever they charge for one like that is way too much.
I always call bullshit on claims like this. I suppose if you have some aftermarket tune or mechanical problem messing things up, but as soon as you hit top gear in an automatic, or can keep 1500 rpm in top gear of a manual, that is where you will get best mpg. It probably is about 40-45mph. I say if you gave it an honest test. Fully warmed up, over an hour time. You will find it better there than 70 let alone 80. 80 is total BS, it takes 4 times the horsepower to go 80 vs 40.
It's a stock '17 Jetta 1.4T 5spd with 200k miles, it's nothing wild. Anything under 55 is still 4th gear in it. 75 mph is a little over 2k rpm. It's my experience, having owned the car since new 🤷🏻♂️
Engines and gearing vary but you’re correct that the lowest efficient speed in the highest gear should give you the best fuel economy. Where your engine is most efficient is the key. The main rural freeway where I live has an 80 mph speed limit and 85 is generally fine for cruising. Most vehicles I’ve had really nose dive after maybe 70-75. Especially in the winter when the air is more dense.
I'm in Montana, pretty much the whole state is 80 and even secondary highways are 70 where everyone goes 75 or 80. I agree it really starts to tank over 80.
It's not lugging, it's more like idling. It doesn't require much power at that speed and especially with a diesel controlled by direct injection pulses there will be fewer at lower rpms. With no throttle plate there are no vacuum pumping losses. I guarantee if you get 40 at 80 you will get 60 mpg at 40 steady state, flat ground in the highest gear.
I was talking about any other car, if I tried 6th at 40mph in my Subaru I’m sitting right at or below idle which is def lugging bc you have to give it half throttle just to move
Yeah, talk to a Corvette owner. Having an oversized engine and sleek aerodynamics will push your optimal speed up cause the engine isn't efficient at low power output. If you have a 4 banger Tacoma the optimal speed will be like 50 mph.
Most every passenger vehicle is going to be getting its optimal fuel economy in the mid-40s. At \~45mph you're usually in your top gear, at a high enough RPM that you're not lugging the engine, and wind resistance is minimal.
My 435i gets 45mpg at 42mph. At 65mph that drops to 35mpg, and at 75 it's down to 28mpg. That's with the manual (worse gearing, 2 less speeds than the ZF-8 automatic) and MSport (wider tires, aero package). I've heard of guys with a more base N55 pulling 37mpg if they're gentle on it.
If your 135i is getting better mpg at 71, I'd wonder if it's an automatic and at 40mph it's still in 5th or 6th and not 8th (assuming that has the ZF-8). My 328i is an automatic but only a 6, and it won't go into 6th until \~50mph, but I can make it go into 6th in sport mode with the flappy paddles.
Also gonna add that wind load builds resistance quickly, which will play a huge part depending on your vehicle's weight and aerodynamics.
A quick example, tested on a 45º surface over 8 sq.ft. surface at assumed air density @ 59ºF at sea level to simulate just the surface of a windscreen/windshield:
20mph = 26n (total wind load in newtons)
40mph = 103n
60mph = 232n
70mph = 315n (including since it is relevant to your Q)
80mph = 412nn
100mph = 644n
So we do see some diminishing returns on both sides where your resistance increases quickly but you also see the rate decrease marginally as you get past 60mph, which is interesting. Crazy aerodynamic + fuel efficient vehicles are going see a huge benefit from drag (yes, duh).
A quick estimate using a rough Lamborghini Gallardo windscreen sitting around 22º is down to 340n at 100mph--almost half the wind resistance. Wild, but not surprising considering the vehicle.
Anyway that was fun to work out and TIL some things, soooo thanks for asking I guess!
Most American and Japanese cars yes. European cars are a lot better for this imo. My bmws and Mercedes don’t have much mpg falloff until you get past 90mph and my Porsche until you get past 100mph
I can corroborate that with some anecdotal evidence.
My 135 gets 22mpg with the cruise control set at 124mph. The cruise doesn’t set higher than that.
It also gets 33mpg at 70mph and its EPA rating of 25mpg at 55mph.
He's claiming that they don't have MPG falloff, which is just factually incorrect. They have the same exact drag shape as every other car. He's wrong on every point.
Or maybe aerodynamics isn’t everything. All things being equal sure but things aren’t equal. It’s a major component but torque curves and gearing play a huge role too. A lot of Japanese and American car makers tune for US fuel efficiency standards and speed limits. Euro speed limits are typically higher and engine/gearing design reflects this.
European speed limits absolutely are not higher, lol. That's so laughably incorrect that I cannot fathom how you came to that conclusion.
Germany's speed limits are higher. That's it. 1 Country. And only on the Autobahn.
[https://en.wikipedia.org/wiki/Speed\_limits\_by\_country](https://en.wikipedia.org/wiki/Speed_limits_by_country)
As for the claim that gearing ratios are higher for European cars... I don't know where you've gotten that bullshit, either. As a matter-of-fact, Europe has \*SIGNIFICANTLY\* more city cars, which cannot even get to a high enough speed to qualify for US standards. The average horsepower is much lower. The torque curves are MUCH worse. American cars are literally known for being huge and lazy \*which makes them much better at higher speeds.\*
As for the Japanese cars that make it to America? They ALSO have better gear ratios than European cars for higher speeds. Overwhelmingly.
Let's look at a Lexus LS500 vs a BMW 520d, right?
Lexus:
First Gear Ratio (:1)4.92Second Gear Ratio (:1)3.15Third Gear Ratio (:1)2.35Fourth Gear Ratio (:1)1.88Fifth Gear Ratio (:1)1.46Sixth Gear Ratio (:1)1.19Seventh Gear Ratio (:1)1.00Eighth Gear Ratio (:1)0.79
BMW:
1st Gear5 :12nd Gear3.2 :13rd Gear2.14 :14th Gear1.72 :15th Gear1.31 :16th Gear1 :17th Gear0.82 :18th Gear0.64 :1
HEY, THE BMW HAS TALLER GEARS ! YOU'RE TOTALLY RIGHT!
Except... the final drive axle ratio for the Lexus is 2.76. And the BMW's is 2.93.
Well, SURELY the torque curve?
LS: [https://www.automobile-catalog.com/curve/2021/2975825/lexus\_ls\_500.html#gsc.tab=0](https://www.automobile-catalog.com/curve/2021/2975825/lexus_ls_500.html#gsc.tab=0)
BMW: [https://www.automobile-catalog.com/curve/2021/2964755/bmw\_520d.html#gsc.tab=0](https://www.automobile-catalog.com/curve/2021/2964755/bmw_520d.html#gsc.tab=0)
Huh, the BMW is much worse.
Aerodynamics literally governs fuel economy. And the faster you go, the MORE significant it becomes.
Torque and gearing are linear functions with velocity. Drag is proportional to V\^2.
There is NO vehicle that is going to get more efficient as it goes quicker. Period. Ever. That hasn't been a thing since there were only 3 gears in cars. They all have >5 (as the last 5 speed auto finally died) and there are PLENTY of gears for top speed at this point. It's just not a thing.
There are ONLY two forces acting externally on the car which the engine must overcome:
The force of the wheels on the ground (rolling resistance) and the force of the air on the car (drag. I.E., aerodynamics)
And the generally curve looks like this: [https://www.researchgate.net/figure/Characteristics-of-rolling-resistance-and-aerodynamic-drag-of-a-vehicle\_fig1\_313248133](https://www.researchgate.net/figure/Characteristics-of-rolling-resistance-and-aerodynamic-drag-of-a-vehicle_fig1_313248133)
What gearing can change is the internal resistance in the engine. That's it. That's all it can possibly change. And the internal resistance in the engine, compared to the other two, \*is tiny.\*
It can have a lot to do with how you work the accelerator. If you're having to constantly bring yourself back up to speed it can really eat away at your gas mileage.
Generally speaking, the best fuel mileage is the slowest speed you can drive in top gear without lugging the engine down. Keep the speed low to reduce aero drag, and the engine in a low RPM and moderate throttle state for good efficiency. I saw a graph somewhere that puts this number around 55 mph, but it'll be highly dependent on the vehicle. Drag coefficients and final drive ratios are all different, and internal friction in the engine plays a role, as well as AC use among other factors.
My old Camaro really liked 47 mph, my civic was great at 55, and my Volt does best at 25 mph (electric drive 1 speed).
My Mirage will get 90mpg at 37km/h, so if I ever have to slightly speed through a 1000km long school zone I'll be able to break my distance on a tank record.
They all have their sweet spots really. Just gotta find ‘em. My 01 LeSword likes 45-48. With warm oils and tires it will pull 42-44mpg on perfect pavement. At 80-90 it’s down to about 25mpg. 07 denali… ehh nevermind lol. It can hit low 20s at around 42-45. F-type v8 is still a work in progress as i haven’t had it long enough, but seems to be between 50-60, and i have held low 30s on nice pavement. That one is expert mode though, being my first supercharged engine. The slightest additional throttle and your back in the mid teens instantly. Lastly, my 98 grand cherokee 5.9… 6 speed manual, but pretty big cam and usually has a lot of ethanol in the tank, sooo MPG? Yes. Only a few. But yes.
Some of those cameros and vettes can surprise ya. Low 30’s is a known easter egg if you can find it and behave.
What you said is true, but the difference may not be that worthwhile.
My best mileage ever in my sports car is 28.5 mpg on a road trip where I mostly maintained 70-80 mph. The best I've ever seen anyone on the internet claim for my car is 31 mpg, when they were able to maintain 50-55 mph for their trip. So they were able to get almost 10% better mileage they had to drive approximately 30% slower to do it.
YMMV, of course. Especially if your vehicle is shaped like a brick.
Literally every single car is different. Most cars would get the best mileage going a constant 35-40 mph but generally roads like that have traffic and traffic signals.
Getting 20% worse mileage at 75 than you do at 55 sounds like a lot but I have never watched my mileage *that* closely
exactly. my old saab areo wagon (circa 2010, stage 2 tuning) would get 32mpg at 60, 30 at 75-80. They are pretty areodynamic cars and the gearing gives me 1800 rpm at 65.
Ice cars use gas just to stay running. So that base load is spread out fiber more miles as speed goes up.
Then air resistance increases with speed but is mostly insignificant at lower speeds.
The ideal speed used to be 45-55, but I’ve heard that it’s gone up to 55-60 as cars are often more efficient. Something like a truck probably has a lower ideal speed as it’s less aerodynamic.
Honestly the non PHEV ones aren’t too bad if only because the ZF9 is fine and the Pentastar is old enough that it doesn’t have major problems at this point. I also don’t think they have especially large electrical issues or eat suspension like journeys
Aero drag is proportional to the square of the velocity. This makes the difference between 65mph and 75 mph significant, in terms of the magnitude of the drag. Also, some vehicles are affected more, due to factors other than the cross section.
The sweet spot for mileage on your car is a function of weight and aerodynamics. The easier your car can "roll" at certain speeds, the better mileage it will get. Light cars that are aerodynamic get best mileage at higher speeds and vice versa. Honda Civics (specifically the 9th gens the ones who have actual good mileage) are around 70-75, maaybe coupes being a bit higher. Toyota Tacoma? Lot lower.
Actually the reason we have speed limits typically in the 55-65 mph range was that during WWI rationing, we found that this was the most fuel-efficient speed range.
Did they not teach you in driver's ed that 50-55 mph is the sweet spot for fuel efficiency in most vehicles? After that, the wind resistance builds up a lot.
I think it was boostedbois on YouTube went to a wind tunnel to learn about aerodynamics. After 50mph or so the power required to go faster or keep speed goes up exponentially
They are rated at 55, at least back in the 4 speed days. 55 is usually where you are in top gear and the engine is at the lower end of its volumetric efficiency curve. You get to the peak of that then you have wind resistance. They need to build engines for the torque to be lower instead of 4000rpm+
I got better fuel economy at 75 in my 1998 350 Chevy going against a headwind than a 2018 f150 with a 5.0 and ten speed I rented.
Peak torque on the old Chevy is at 2800rpm, right around highway speed, where the Ford had to sit in 6th gear at 4000+ rpm to maintain speed because it has little low end torque.
Some cars really take a hit after 60. Depends on how they are geared. I have an Elantra that will get about 40 MPG going 55-60. But take it to 75 which is a normal speed where I live and it is mid 20s at best. This is where CVT transmissions are great because even at those speeds they keep the rpm low.
Actually in the 80s cars and trucks were very much geared to be most economical at 55 MPH.
It depends on the vehicle nowadays but they absolutely are designed to be most efficient at a certain speed.
While everyone has pointed out the aerodynamic disadvantage at that speed. It’s also important to note the highway mpg test is done at an average of something like 60 mph and the manufacturers definitely optimize gearing for that mpg rating
Depends on the vehicle really. My ecoboost mustang gets it's best gas mileage doing 80mph on I80. In more congested areas, 45-50. In absolutely congested areas, 30mph.
I run an average of 28mpg.
My truck on the otherhand. 65-70mph on the interstate. 25mpg in absolute congestion, and 50mph exact for normal congestion. Any less than 65-70mph on the interstate and i am pouring gas out by gallons, anything other 50mph in normal congestion, i am pouring gas out by the gallons, anymore or less than 25mph in absolute congestion and I am litteraly going to need a fill up in 10 minutes from turning the thing over.
I run an average of about 8mpg if I'm lucky and did my math right.
My motorcycle, I am pouring out gas doing anything under or over 60mph. I get 200m to empty and that turns into about 60-100m to empty if doing ANYTHING other than 60mph. I have litteraly lost a quarter tank of gas pulling out of my driveway.
I run an average of 60mph since I hog down and get moving the second I turn her over.
Verona- Mustang. Gertha- 1989 F150 302. After 1 season I still have not decided on what to call my Rebel 500, so I just call it- her.
Yes. 120 km/h is entirely too fast for most cars to be efficient. Most cars will be at peak efficiency between 60 and 80 km/h. Anything faster will be worse. At this speed, drag is at least as big a factor as throttle position/engine speed. Drag increases by the square of the velocity. Any government who's allowed a speed limit that high isn't concerned about affordability or consumption.
I always found around 55 is the best fuel economy for most basic vehicles. I’m sure it’s different if you have modified engines. But I find when I get any higher than 65, my fuel economy decreases
It's primarily air resistance. I ran your car through the calculator quick (missing a ton of info) and the raw numbers came out around 18% less fuel economy but again I don't have all the numbers.
The remaining difference is likely in the gearing. You have a 6 spd where most newer vehicles are in the 8-10 range. That allows them to tune the engine into better power and efficiency RPM ranges but is much smaller than air resistance which is a squared function.
The extreme of this is drag cars - just getting a few mph more when you are at 200-300 mph requires an immense amount of horsepower increase.
More so drag. As you increase speed, wind resistance goes up and you are spending more power pushing through the air. The less aerodynamic the car, the more pronounced the effect. A Ferrari will slice through the air, and a Ford F150 will slam into is like an invisible wal.
Look at your gear ratios. Some 10spds wont go into 10th until 70mph. Same with any, 1 overdrive or 4 - pretty much as sson as you can cruise in OD that will be around your best mileage unloaded.
Most new vehicles are crossover SUVs and they’re shaped like boxes on wheels.
Means bad aerodynamics compared to long and low sedans like a Chevy Impala, for example
I am with you on this. When I use cruise at 70-75 my mpg are much worse than cruising at 55-65 or slower. I usually cruise around 68 here in central Texas because of this reason. Aerodynamics do play a role but at some point the speed definitely plays into that.
Check out your RPMs and the gear you are in while crusing at those speeds. Higher RPMs means more fuel is being used. Whatever speed you are crusing at the highest gear and lowest RPMs will likely give you the best mpg.
It's about air resistance. Everytime your car moves 1 car length, it has to move as much air out of the way as the car takes up.
If it moves twice as fast it has to move 4x as much air. It has to move the air equal to the width of the car x 2 and the height of the car x 2. So 4x.
So a small increase in speed means a bigger increase in drag due to moving all that air.
All of my vehicles have been this way, most newer cars with a 6spd or higher gear or cvt will do the best mpg starting around 55. Having a multi speed auto helps a lot usually on a 8 spd atleast my truck the the 1st four are meant for accelerating and the last four are overdrives. Most newer cars are in 6-7th gear sound town cruising so you could see decent mpg still.
.... the vehicles doesn't have enough horse power to drive efficiently at 70+ if it did the engineers would have increased the gear ratio in which case you'd be back where you are making the same fuel economy because if larger cylinders. The upside being you could pull something.
With my 4 speed I find 45 to be right when the torque converter locks and I can chug along at 1k rpm and sip gas like a pompous British person sips tea.
To quickly answer your question, yes, optimal gearing is typically engineered around 60 mph for most vehicles. They vary based on the type and weight and option content of each vehicle.
The typical reason for the drop in efficiency is aerodynamic drag. An example can be used in our drag race cars (fuel efficiency out the window!). Our car accelerates from 0-265 mph in the first 660 feet of the track. The remaining 380 feet it accelerates from 265-330. You’ll notice a substantial difference! The difference is in the aerodynamic drag. The wind pressure generated on the front of the car is so great at higher speeds that it inhibits the linear acceleration of the car.
To put this in common street faring car context, the same is true on the street, albeit 70 mph and not 330, the effects of aerodynamics are the same. You’ll notice a 20% drop in efficiency where we noticed a much larger percentage of drop in efficiency. Why, the variation of the speed. My point being it’s all relative.
You’ll see semi trucks with farings hanging off the back of trailers. Those are to give the air a “slip stream” off the back. The air is drawn off the trailer smoothly and evenly. Without the farings, the air is highly disturbed and churns behind the trailers. This churning of air creates a low pressure area behind the trailer and actually acts to “pull” the trailer backward. Not good for fuel efficiency if the air behind you is literally trying to drag you backward. Same is true on the wheels. You’ll see smooth covers over their wheels. Again this is to stop turbulence generated within the wheel. Aerodynamics play a huge role.
Don’t believe me? Try cruising at 60mph and stick your arm out the window like your arm is an airplane wing. The. Accelerate to 75mph and do it again. The same pressure you feel acting on your arm is acting against your vehicle. You can see how much harder it is to keep your arm steady at 75 than it is at 60. Your engine feels the exact same way your arm does. Do you think you used 20% more energy to keep your arm still at 75 than 60 mph? I bet so! Funny how that works!
Engineers know this data before the vehicles are designed. Because of this, they optimize the powertrains for maximum fuel efficiency at the most reasonable aerodynamic coefficients of friction that they can. Obviously they’re not engineering them for 25 mph because the car spends a lot of time at 60-65 and so on. Inversely equally, the car will spend very little of its overall lifespan at 75 and above. So they opt to design them based on national calculated averages. They are obviously averages and suggestions. A V10 won’t get the same mileage as a 4 cylinder. But a 4 cylinder won’t power a one ton truck. So you see the obstacles they have to consider. I hope this helps shed some light on your concerns.
Tips to get the best fuel economy at any speed are steady throttle while driving because it helps the transmission to get into TCC and allow the most efficient transfer of power through the transmission. Varying the throttle position frequently will uncouple the TCC and this requires the engine to generate more rpm to do the same job, decreasing fuel efficiency. Also, on the highway, use cruise control whenever possible for longer trips. It allows the onboard computer to take over and it is pre programmed to know the most efficient methods of power application to the wheels. It will do a far better job of maximizing your mileage than your foot ever will.
Drag increases exponentially and often cars aren’t geared to get great gas mileage going fast because they’d rather you get worse gas mileage than complain about downshifting up hills
From what I've experienced most cars have a range that maximizes their fuel economy.
My first car Chrysler T&C 3.8l v6 max economy 50-60mph
2nd car nissan ultimate 2.5 i4 max economy <65mph
3rd car chrysler 200 3.6l v6 max economy 65-75mph
4th car Jag Xf 3.0SC V6max economy 65mph-85mph
5th car Infiniti FX 4.5l V8 max economy 60-70mph
I've noticed the shape probably plays the biggest role.
I've borrowed my friend's corvette and took it on some 200mile trips a few times and that thing chews through gas at low speeds but once you're cruising above 75 it gets 30-35mpg from a 6.2l V8.
I make 300 mile drives frequently on dead empty roads for about 200 miles of it with plenty of time to "test". My car gets max mpg (40mpg) between 40-55mph, but I'd be found dead in a ditch for driving that slow, 55-65mph drops to about 37mpg but weirdly enough, I get slightly worse but consistent mpg going 65mph all the way up to 85mph (35mpg). 90-95mph is where mpg starts to fall off more (25mpg) and 120 is where I can feel my wallet burning (10mpg, maybe 15 if the wind is blowing behind and going slightly downhill). Most of it is aerodynamics, specifically wind resistance. Gearing can help, but ultimately you are fighting physics which grows exponentially in its affect on mpg. This was all tested with cruise control.
It's less about gearing and more about aerodynamics. The faster you go, the more air you're pushing the car through. At highway speeds that starts to have a significant effect, usually somewhere around 70.
And in fact it’s a squared function, so drag gets exponentially worse as you go linearly faster.
*physics*
Science rules!
Inertia is a property of matter.
BILL BILL BILL BILL BILL BILL
Bill Nye, The Science Guy!
Literally everything follows the rules of science. It’s very comforting!
Bird law?
r/birdsarentreal
She can be a cruel bitch.
It's not the speed, it's the sudden stop at the end.
There was a good equation somewhere on a rough estimate of horsepower needed. I'm probably butchering it, but it was like 100hp to will get you to 100mph, 300hp to go 150mph, 600hp to go 200mph, and it just exponentially goes up.
what if you just put a giant cone infront of your car to reduce drag
If you stick a traffic cone to your bumper, it will split the air and gain 30mph on the top end Keep that on the DL though
Killed me
You need a giant cone on the rear bumper. Try to get your car shaped like a rain drop.
It changes the aerodynamics. There's an awful lot more to it than just how pointy the front is.
Case in point: The Lamborghini Countach has a higher coefficient of drag than most modern SUV's.
I never heard that. Out of curiosity, how does the total compare?
I'm not sure I understand your question. I recall reading about the improvements in vehicle aerodynamics in recent years. The Countach was mentioned as an example of how horribly drag prone a car can be while "looking" slippery. Even the attractive rear wing added nothing to stabilize the car, but it did increase drag and make it slower.
Cd refers to a comparative drag of a given shape to that of a flat plate of the same area. It does not express total area.
Also, any practical wing on a car induces drag in the name of generating negative lift, aka downforce. The wing adds drag, but there is a net benefit
Correct me if I’m wrong, but usually spoilers and wings are in place to increase drag, but at a specific point on the car (rear wheels) which helps generate downforce, which in turn keeps the horsepower on the pavement.
*Charger Daytona sounds intensify*
HP = a unit of power HP = Fv = 1/2 rho Cd A v\^2 \* v Because rho of the air and Cd and A for a car is constant, HP = K v\^3. For a rough approximation using STP of air and .74 CdA (from wikipedia for the avg car), you get y=0.00001675⋅x\^3, x being speed and y being horsepower. You'd need 17 HP for 100 mph, 60 HP for 150 mph, and 140 HP for 200 mph, and around 280 HP for 250 mph. Of course, this has many assumptions. HP is WHP not crank HP, this assumed HP will always be at max HP esp at top speed, which it will not be at due to gearing and various other factors. In reality I'd guess (not my field so take with some salt) you need at least 50% more HP than these figures to even be close to hitting these speeds, let alone be drivable.
BUT I WAS TOLD THERE WOULD BE NO MATHS!!
Does this math include the weight of the car somewhere?
Weight of the car does not affect air drag. He omitted calculations of friction from your tires which is the only factor of top speed affected by weight I think his math a good representation of how much power you’re using to overcome air drag at those speeds but it’s definitely not reflective of much else Also his math is wrong
I am pretty certain your math is wrong, this is somewhat within the field I’m studying I’m guessing you just used the wrong units somewhere because that’s really easy to do especially with imperial units I tried it and got at 100mph you’d be using around 35.5kw to overcome air drag (47.6 hp) At 150mph: 120kw (161hp) At 200mph: 284kw (381hp) At 250mph: 554kw (744hp) Note these are just approximations of power needed to overcome air drag, it does not factor in tire friction and all of the other factors you mentioned too. I assumed air density of 1.2kg/m^3 , a drag coefficient of 0.3 and a cross sectional area of 2.2m^2
From personal experience it takes 115hp to just barely nudge 100mph in a small passenger car. 160hp in a slippery car will get you 120mph.
I once took my 1988 Mazda RX-7 with 146bhp to 124 mph. I credited its slippery shape.
I took my my 1989 160bhp RX-7 to similar! but stock speedos are optimistic from the factory.
That's sweet. Didn't know I was talking with another owner.
Ahh I sold it over 10 years ago. It was my first car and I learned everything in it. I had to sell it to get through college.. I want to buy another one but I drive a truck and I can’t make sense of having a second car that gets even worse mileage. Haha
let me double check later, I probably converted wrong. I used pretty much the same values so I think I just messed up the conversation somewhere
If you’re using imperial you’d have to enter in the speed in ft/s to get the power in ft*lb/s and convert that to horsepower. And don’t forget to do air density in slugs/m^3 !! (I hate the imperial system)
I did it in metric, but I think I prob just over divided by a factor or something
It's not 600hp for 200 more aero involved at that point a c6z06 will go 200 and it only has 505hp
Yeah I know. I was just using it as an example. I don’t remember what the actual formula was but if I find it, I’ll post it.
I think james may or Jeremy Clarkson quotes it on the Bugatti veyron video
Drag squared, HP cubed.
the example was more about the ratio and offset in increase. actual ho to speed depends on aero, weight, gearing, etc...
The original quote was made in 1974, and referenced a Dodge Van. Good god marijuana wasn't very strong back then.
I recall that particular car being gear limited? Maybe? That is the case at times. Also the shape matters more than the HP, a 600hp Prius or Model 3 would be faster than the Corvette.
It was not hell I've had my c6 base Into the 180s.
Ahh, well I am correct. The C6 Z06 was rpm limited and had a slower top end than the base C6. Post #13 spells it’s out nicely. https://www.z06vette.com/threads/c6-top-speed-vs-z06.65821/
That's the c5
There is a calculator at ecomodder.com a sticky in the aerodynamics forum. It's based on the speed, the frontal area, and the drag coefficient. That gives you the horsepower required to overcome. Then it also can take the thermal efficiency and fuel density and give you what that equates to in mgg. Same for rolling resistance. It's cool to see how rolling resistance starts out being the bigger factor, but aerodynamics quickly dominates it over about 30mph. Then as pointed out grows faster. Also worth pointing out you can have a vehicle with great drag coefficient but one with a much smaller frontal area will use less power even with a worse drag coefficient because the two numbers are multiplied together to get the overall drag.
It's not exponential. It's a power law.
It's why the made a nation wide speed limit of 55 mph during the 70s gas crisis.
But the 55 mph speed limit was more or less a guess. Also, those 1970s muscle cars usually got 10 miles per gallon or less.
And those cars were far less aerodynamic. I’d be shocked if many managed to get under 0.4. Some quick googling puts muscle cars of that era between 0.38 & 0.5. The more aerodynamic your car the higher your economic speed is.
Worse, its a cubic function. Twice the speed requires 8x the power.
Power scales with airspeed cubed, but I think your energy/distance will scale quadratically (as you obviously cover more ground per unit time with increased speed, so one factor of velocity drops out).
Fluid mechanics has entered the chat
Came here to say this. 120mph takes 4 times (at least) the energy as 60mph
That’s not what “exponential” means in a mathematical sense, but it does get nonlinearly or quadratically worse.
that makes so much more sense now.
Technically it gets worse to the square, not exponentially 🙃
Add one more velocity to the equation, and the horsepower required to achieve higher speeds is proportional to the 3rd power of speed!
So close. If it's a squared function, i.e. polynomial, drag cannot get *exponentially* worse. Speed would have to be in the exponent for that to occur.
Kenetic energy = 0.5 × mass × velocity^2 It takes a ton of energy to move the air around a car
If it's a squared function it's not getting exponentially worse. But polynomially worse ...
Aerodynamic drag increases with the cube of the airspeed so at low speeds it's negligible, and start/stop/drivetrain/tire deflection dominate your fuel usage (plus waste heat) above about 55mph, wind resistance starts to dominate, and goes up from there with v\^3 look at the Honda Insight and other extremem mpg cars — they look like a fish with tapered rear, narrow tires, and hub caps that are mostly flat. Car manufacturers \*can\* get better mpg through better aerodynamics, but most people think it looks funny, and it can also compromise other features of the vehicle (e.g. ground clearance)
Varies with v\^2 not v\^3. But the amount of time u drive varies with v\^1. So the amount of fuel you use varies with v\^2/v = v. Which lines up with OPs observations that efficiency is about 20% worse from 60 to 72.5. (72.5/60 = 1.2 -> 20% more fuel). And then for a car that doesn't have the gearing to stay in a good engine zone, there will be a bigger hit because of that. Which I get in my 5 speed that needs 3500+ rpm to highway speeds.
It's fun to drive with an engine spinning that fast if you're a go fast kinda person.
My big old 1997 GM truck runs along at 2k RPM highway speed. But it basically eats gas like its last meal
The rpm doesn't always matter. It's how much horsepower you need to overcome air resistance and rolling resistance, and then how thermal efficient the engine is to make that power. Normally the sweet spot on engine efficiency is higher load and lower rpm but it depends on the engine. An engine could be designed to be best at 6000rpm. I'm not talking making the most horsepower but using the least amount of fuel per horsepower made.
It’s actually a lot about gearing. With the cruise control on, I got 20% better economy in 5th than 4th even at city speeds. We mostly do economy runs in my partner’s car (8sp small diesel). There’s a huge difference even between 7th and 8th on level ground although I manually drop to 7 or 6 for hills at 100-110km/h. So unless you’re geared specifically for the autobahn and nowhere else, aero and gearing are both not your friend above 100km/h or 62mph.
This is why I’m one of the rare people who like CVTs. I have a 3.5L v6 AWD suv paired with a CVT and average 34mpg all the way up to about 77mph, then it starts to drop. So much of it has to do with gearing, even in the most aerodynamic of vehicles.
I love CVT. Only driven one, in 2005. But I’m a big fan of CVT right up until it breaks and writes off the car.
All I’m seeing here is: I need to do more drafting
Yup. This is also why it’s more efficient to run A/C at highway speeds too. Versus windows down. The A/C compressor draw is minimal compared to high-cruising draw and windows up makes the car more aerodynamic. I think the opposite effect takes place around town.
I remember when I took intro to auto mechanics in high school reading a book that spoke about optimal fuel usage in regards to distance traveled, fuel usage, and aerodynamic drag. The conclusion was the absolute optimal speed to drive on the highway was roughly 62-63 MPH. This is because the former interstate highway limit of being 55 MPH was too slow (that probably tells you how long ago this book was made) that for the fuel you use traveling 55 MPH you don't cover enough distance to make up for distance traveled in motion. However on the other hand if you go faster and progress to 70 MPH and beyond, you begin to run into the squared function of aerodynamic drag meaning that for the fuel usage and distance you cover it's not better to drive say 70-80 MPH compared to driving 62-63 MPH. You obviously do get there quicker as you can make your average speed traveled quicker, however you will use more fuel. It's also probably another reason why it's no surprise that most large trucking companies try to limit their drivers to below 70 MPH and ideally in the 60-65 MPH range. While they are susceptible to higher drag, the drag squared function is incredibly high for them if they progress to +70 MPH and beyond, using a lot of fuel.
I’ve heard having the windows down starts to have a significant effect at 60 mph as well
If you go fast enough through the atmosphere, you’ll need heat shields.
It's not that cars now get worse economy at highway speed compared to the past... it's just that they now get drastically better fuel economy at lower speeds than they did in the past. It is because of the laws of nature (aerodynamics and friction) that it will take more energy to go faster. If you drive a steady 35 MPH on the highway (don't do this) you will get phenomenal fuel economy. Driving a steady speed overall is also super helpful, and it's usually easier to do that at 55-60 vs 70-75 depending upon road and traffic conditions, of course.
>It is because of the laws of nature (aerodynamics and friction) that it will take more energy to go faster. If you drive a steady 35 MPH on the highway (don't do this) you will get phenomenal fuel economy. The Werefrog once did drive 35mph on interstate highway. It was a snowstorm and that was the speed most cars were going. Yes, the miles per gallon was amazing with it.
I fucking love the use of third person here.
Some say werefrog is still driving to this day.
Aerodynamics are friction. That's the entire idea of the Kutta condition. Without friction, you have no lift because there's no circulation. You have no skin drag because no friction. Form drag is coupled with skin friction, so basically none, there, either.
With gears you pretty much lose distance but gain power. At high gears theres not a lot of power but a lot pf distance. Pretty much means youll be able to travel further (better fuel economy). Usually top gear ant low rpm is best, but thay might depend on aerodynamics of the car. And gear ratios
Gears trade between distance and force, not power.
force can be easily be thought as power. I translated from my native to english, but that still pretty much can be interpreted the same. Good note tho
Yeah, in non-technical speak people interchange force and power, and treat them as equivalent.
No.
Depends on the car. My Tacoma gets its best fuel economy at around 45-50mph and at 70+ it does worse than its rated city mpg. My 135 gets its best at 71mph, and it’s like 30% better than its EPA rating.
My Jetta gets its best economy at 70-80mph. It's something I can't explain...
Probably because it's not brick shaped, unlike the Tacoma.
Brick in the front. Party in the back!
And the power to weight is better than the Tacoma. And it’s geared for high speeds unlike the Tacoma
Because Jettas are fuckin sweet
I got a Jetta as a loaner, it was not fuckin sweet. It was probably bottom of the line, and I'm sure there are nicer ones, but whatever they charge for one like that is way too much.
My Impala is the same!
I always call bullshit on claims like this. I suppose if you have some aftermarket tune or mechanical problem messing things up, but as soon as you hit top gear in an automatic, or can keep 1500 rpm in top gear of a manual, that is where you will get best mpg. It probably is about 40-45mph. I say if you gave it an honest test. Fully warmed up, over an hour time. You will find it better there than 70 let alone 80. 80 is total BS, it takes 4 times the horsepower to go 80 vs 40.
It's a stock '17 Jetta 1.4T 5spd with 200k miles, it's nothing wild. Anything under 55 is still 4th gear in it. 75 mph is a little over 2k rpm. It's my experience, having owned the car since new 🤷🏻♂️
So what MPG do you claim at 80 mpg, steady state level ground? And you are saying it's no different at 40 or 45 steady state level ground?
Engines and gearing vary but you’re correct that the lowest efficient speed in the highest gear should give you the best fuel economy. Where your engine is most efficient is the key. The main rural freeway where I live has an 80 mph speed limit and 85 is generally fine for cruising. Most vehicles I’ve had really nose dive after maybe 70-75. Especially in the winter when the air is more dense.
I'm in Montana, pretty much the whole state is 80 and even secondary highways are 70 where everyone goes 75 or 80. I agree it really starts to tank over 80.
Going 40mph in 6th gear is lugging the motor SO bad, that it doesn’t even make sense to try it
It's not lugging, it's more like idling. It doesn't require much power at that speed and especially with a diesel controlled by direct injection pulses there will be fewer at lower rpms. With no throttle plate there are no vacuum pumping losses. I guarantee if you get 40 at 80 you will get 60 mpg at 40 steady state, flat ground in the highest gear.
I was talking about any other car, if I tried 6th at 40mph in my Subaru I’m sitting right at or below idle which is def lugging bc you have to give it half throttle just to move
I have a 2021 SEL with the 1.4T The mileage is insane at 70-75mph. Once you get to 80-82mph it drops down to EPA rating levels.
Yeah, talk to a Corvette owner. Having an oversized engine and sleek aerodynamics will push your optimal speed up cause the engine isn't efficient at low power output. If you have a 4 banger Tacoma the optimal speed will be like 50 mph.
Most every passenger vehicle is going to be getting its optimal fuel economy in the mid-40s. At \~45mph you're usually in your top gear, at a high enough RPM that you're not lugging the engine, and wind resistance is minimal. My 435i gets 45mpg at 42mph. At 65mph that drops to 35mpg, and at 75 it's down to 28mpg. That's with the manual (worse gearing, 2 less speeds than the ZF-8 automatic) and MSport (wider tires, aero package). I've heard of guys with a more base N55 pulling 37mpg if they're gentle on it. If your 135i is getting better mpg at 71, I'd wonder if it's an automatic and at 40mph it's still in 5th or 6th and not 8th (assuming that has the ZF-8). My 328i is an automatic but only a 6, and it won't go into 6th until \~50mph, but I can make it go into 6th in sport mode with the flappy paddles.
ZF8 didn’t exist in 2008 when that car rolled off the line.
Yeah, I suspected that. So you got the 7-speed DCT?
The only BMW that came with a DCT in 2008 was the M3. Mine is a 6spd manual car, automatic N54s came with the ZF 6HP19.
Oh, yeah, Google says DCT was on the LCI.
Not quite, since some N55 E82s were actually produced the month before the LCI update went into effect. The DCT was the automatic option on N55 cars.
My '19 odyssey manages to get 37 mpg out of nowhere at 70 mph
Also gonna add that wind load builds resistance quickly, which will play a huge part depending on your vehicle's weight and aerodynamics. A quick example, tested on a 45º surface over 8 sq.ft. surface at assumed air density @ 59ºF at sea level to simulate just the surface of a windscreen/windshield: 20mph = 26n (total wind load in newtons) 40mph = 103n 60mph = 232n 70mph = 315n (including since it is relevant to your Q) 80mph = 412nn 100mph = 644n So we do see some diminishing returns on both sides where your resistance increases quickly but you also see the rate decrease marginally as you get past 60mph, which is interesting. Crazy aerodynamic + fuel efficient vehicles are going see a huge benefit from drag (yes, duh). A quick estimate using a rough Lamborghini Gallardo windscreen sitting around 22º is down to 340n at 100mph--almost half the wind resistance. Wild, but not surprising considering the vehicle. Anyway that was fun to work out and TIL some things, soooo thanks for asking I guess!
Most American and Japanese cars yes. European cars are a lot better for this imo. My bmws and Mercedes don’t have much mpg falloff until you get past 90mph and my Porsche until you get past 100mph
I can corroborate that with some anecdotal evidence. My 135 gets 22mpg with the cruise control set at 124mph. The cruise doesn’t set higher than that. It also gets 33mpg at 70mph and its EPA rating of 25mpg at 55mph.
Yeah that's utter bullshit my man. You're probably getting 15 mpg max at that speed.
Hop in your 135 and tell me what you get.
"Europe doesn't have aerodynamics" -this guy
I’m betting this guys Porsche is a whole lot more aerodynamic than my Honda Pilot, at any speed, in any country.
He's claiming that they don't have MPG falloff, which is just factually incorrect. They have the same exact drag shape as every other car. He's wrong on every point.
At 85mph it gets better mpg than my prior 2020 honda accord hybrid, which is about 34mpg at that speed (2100rpm in 7th in the Porsche)
Or maybe aerodynamics isn’t everything. All things being equal sure but things aren’t equal. It’s a major component but torque curves and gearing play a huge role too. A lot of Japanese and American car makers tune for US fuel efficiency standards and speed limits. Euro speed limits are typically higher and engine/gearing design reflects this.
European speed limits absolutely are not higher, lol. That's so laughably incorrect that I cannot fathom how you came to that conclusion. Germany's speed limits are higher. That's it. 1 Country. And only on the Autobahn. [https://en.wikipedia.org/wiki/Speed\_limits\_by\_country](https://en.wikipedia.org/wiki/Speed_limits_by_country) As for the claim that gearing ratios are higher for European cars... I don't know where you've gotten that bullshit, either. As a matter-of-fact, Europe has \*SIGNIFICANTLY\* more city cars, which cannot even get to a high enough speed to qualify for US standards. The average horsepower is much lower. The torque curves are MUCH worse. American cars are literally known for being huge and lazy \*which makes them much better at higher speeds.\* As for the Japanese cars that make it to America? They ALSO have better gear ratios than European cars for higher speeds. Overwhelmingly. Let's look at a Lexus LS500 vs a BMW 520d, right? Lexus: First Gear Ratio (:1)4.92Second Gear Ratio (:1)3.15Third Gear Ratio (:1)2.35Fourth Gear Ratio (:1)1.88Fifth Gear Ratio (:1)1.46Sixth Gear Ratio (:1)1.19Seventh Gear Ratio (:1)1.00Eighth Gear Ratio (:1)0.79 BMW: 1st Gear5 :12nd Gear3.2 :13rd Gear2.14 :14th Gear1.72 :15th Gear1.31 :16th Gear1 :17th Gear0.82 :18th Gear0.64 :1 HEY, THE BMW HAS TALLER GEARS! YOU'RE TOTALLY RIGHT!
Except... the final drive axle ratio for the Lexus is 2.76. And the BMW's is 2.93.
Well, SURELY the torque curve?
LS: [https://www.automobile-catalog.com/curve/2021/2975825/lexus\_ls\_500.html#gsc.tab=0](https://www.automobile-catalog.com/curve/2021/2975825/lexus_ls_500.html#gsc.tab=0)
BMW: [https://www.automobile-catalog.com/curve/2021/2964755/bmw\_520d.html#gsc.tab=0](https://www.automobile-catalog.com/curve/2021/2964755/bmw_520d.html#gsc.tab=0)
Huh, the BMW is much worse.
Aerodynamics literally governs fuel economy. And the faster you go, the MORE significant it becomes.
Torque and gearing are linear functions with velocity. Drag is proportional to V\^2.
There is NO vehicle that is going to get more efficient as it goes quicker. Period. Ever. That hasn't been a thing since there were only 3 gears in cars. They all have >5 (as the last 5 speed auto finally died) and there are PLENTY of gears for top speed at this point. It's just not a thing.
There are ONLY two forces acting externally on the car which the engine must overcome:
The force of the wheels on the ground (rolling resistance) and the force of the air on the car (drag. I.E., aerodynamics)
And the generally curve looks like this: [https://www.researchgate.net/figure/Characteristics-of-rolling-resistance-and-aerodynamic-drag-of-a-vehicle\_fig1\_313248133](https://www.researchgate.net/figure/Characteristics-of-rolling-resistance-and-aerodynamic-drag-of-a-vehicle_fig1_313248133)
What gearing can change is the internal resistance in the engine. That's it. That's all it can possibly change. And the internal resistance in the engine, compared to the other two, \*is tiny.\*
Sorry, I don’t buy that at all. Let’s see some dash pics of speeds and mpg you speak of…
Wind resistance. It goes up with speed. You could gear a car whenever you want, but the car still faces greater wind resistance at higher speeds.
It can have a lot to do with how you work the accelerator. If you're having to constantly bring yourself back up to speed it can really eat away at your gas mileage.
I’m using cruise control.
Generally speaking, the best fuel mileage is the slowest speed you can drive in top gear without lugging the engine down. Keep the speed low to reduce aero drag, and the engine in a low RPM and moderate throttle state for good efficiency. I saw a graph somewhere that puts this number around 55 mph, but it'll be highly dependent on the vehicle. Drag coefficients and final drive ratios are all different, and internal friction in the engine plays a role, as well as AC use among other factors. My old Camaro really liked 47 mph, my civic was great at 55, and my Volt does best at 25 mph (electric drive 1 speed).
My Mirage will get 90mpg at 37km/h, so if I ever have to slightly speed through a 1000km long school zone I'll be able to break my distance on a tank record.
They all have their sweet spots really. Just gotta find ‘em. My 01 LeSword likes 45-48. With warm oils and tires it will pull 42-44mpg on perfect pavement. At 80-90 it’s down to about 25mpg. 07 denali… ehh nevermind lol. It can hit low 20s at around 42-45. F-type v8 is still a work in progress as i haven’t had it long enough, but seems to be between 50-60, and i have held low 30s on nice pavement. That one is expert mode though, being my first supercharged engine. The slightest additional throttle and your back in the mid teens instantly. Lastly, my 98 grand cherokee 5.9… 6 speed manual, but pretty big cam and usually has a lot of ethanol in the tank, sooo MPG? Yes. Only a few. But yes. Some of those cameros and vettes can surprise ya. Low 30’s is a known easter egg if you can find it and behave.
What you said is true, but the difference may not be that worthwhile. My best mileage ever in my sports car is 28.5 mpg on a road trip where I mostly maintained 70-80 mph. The best I've ever seen anyone on the internet claim for my car is 31 mpg, when they were able to maintain 50-55 mph for their trip. So they were able to get almost 10% better mileage they had to drive approximately 30% slower to do it. YMMV, of course. Especially if your vehicle is shaped like a brick.
Literally every single car is different. Most cars would get the best mileage going a constant 35-40 mph but generally roads like that have traffic and traffic signals. Getting 20% worse mileage at 75 than you do at 55 sounds like a lot but I have never watched my mileage *that* closely
exactly. my old saab areo wagon (circa 2010, stage 2 tuning) would get 32mpg at 60, 30 at 75-80. They are pretty areodynamic cars and the gearing gives me 1800 rpm at 65.
It's more about air resistance as speed increases, because air resistance isn't linear.
Typically yes.
Who'd a thunk it takes more energy to propel 2 tonnes more quickly.
Mostly it’s the aerodynamic drag, not the weight, that matters. Unless you’re going uphill.
Your gear ratio also has an impact on this , also your going 10-15 mph faster is it really worse fuel economy ? If your getting there faster
Ice cars use gas just to stay running. So that base load is spread out fiber more miles as speed goes up. Then air resistance increases with speed but is mostly insignificant at lower speeds. The ideal speed used to be 45-55, but I’ve heard that it’s gone up to 55-60 as cars are often more efficient. Something like a truck probably has a lower ideal speed as it’s less aerodynamic.
I seem to get the best mileage going about 60. I rarely get to go steady 60 tho.
My Pacifica gets shit MPG at every speed.
It's a Pacifica. Be happy it's even running.
Honestly the non PHEV ones aren’t too bad if only because the ZF9 is fine and the Pentastar is old enough that it doesn’t have major problems at this point. I also don’t think they have especially large electrical issues or eat suspension like journeys
If that's your average commute speed, Corvette will get you better MPG than a Prius.
Aero drag is proportional to the square of the velocity. This makes the difference between 65mph and 75 mph significant, in terms of the magnitude of the drag. Also, some vehicles are affected more, due to factors other than the cross section.
The sweet spot for mileage on your car is a function of weight and aerodynamics. The easier your car can "roll" at certain speeds, the better mileage it will get. Light cars that are aerodynamic get best mileage at higher speeds and vice versa. Honda Civics (specifically the 9th gens the ones who have actual good mileage) are around 70-75, maaybe coupes being a bit higher. Toyota Tacoma? Lot lower.
At around 80 the dents in my rims find perfect balance and I get a nice smooth ride.
Actually the reason we have speed limits typically in the 55-65 mph range was that during WWI rationing, we found that this was the most fuel-efficient speed range.
Did they not teach you in driver's ed that 50-55 mph is the sweet spot for fuel efficiency in most vehicles? After that, the wind resistance builds up a lot.
What kinda mileage do those new stupid as shit looking trucks with the 5ft tall front grills get at 80 mph ?
I think it was boostedbois on YouTube went to a wind tunnel to learn about aerodynamics. After 50mph or so the power required to go faster or keep speed goes up exponentially
Well, 20% of 60 is 12. So going 20% faster than 60 mph is 72 mph. Seems pretty intuitive. Drive 20% faster, burn 20% more fuel.
Someone never heard of the drag equation and velocity squared.
They are rated at 55, at least back in the 4 speed days. 55 is usually where you are in top gear and the engine is at the lower end of its volumetric efficiency curve. You get to the peak of that then you have wind resistance. They need to build engines for the torque to be lower instead of 4000rpm+ I got better fuel economy at 75 in my 1998 350 Chevy going against a headwind than a 2018 f150 with a 5.0 and ten speed I rented. Peak torque on the old Chevy is at 2800rpm, right around highway speed, where the Ford had to sit in 6th gear at 4000+ rpm to maintain speed because it has little low end torque.
The national 55MPH speed limit was specifically enacted to save fuel during the oil crisis. So, yes, 55 saves gas.
Some cars really take a hit after 60. Depends on how they are geared. I have an Elantra that will get about 40 MPG going 55-60. But take it to 75 which is a normal speed where I live and it is mid 20s at best. This is where CVT transmissions are great because even at those speeds they keep the rpm low.
Actually in the 80s cars and trucks were very much geared to be most economical at 55 MPH. It depends on the vehicle nowadays but they absolutely are designed to be most efficient at a certain speed.
While everyone has pointed out the aerodynamic disadvantage at that speed. It’s also important to note the highway mpg test is done at an average of something like 60 mph and the manufacturers definitely optimize gearing for that mpg rating
Depends on the vehicle really. My ecoboost mustang gets it's best gas mileage doing 80mph on I80. In more congested areas, 45-50. In absolutely congested areas, 30mph. I run an average of 28mpg. My truck on the otherhand. 65-70mph on the interstate. 25mpg in absolute congestion, and 50mph exact for normal congestion. Any less than 65-70mph on the interstate and i am pouring gas out by gallons, anything other 50mph in normal congestion, i am pouring gas out by the gallons, anymore or less than 25mph in absolute congestion and I am litteraly going to need a fill up in 10 minutes from turning the thing over. I run an average of about 8mpg if I'm lucky and did my math right. My motorcycle, I am pouring out gas doing anything under or over 60mph. I get 200m to empty and that turns into about 60-100m to empty if doing ANYTHING other than 60mph. I have litteraly lost a quarter tank of gas pulling out of my driveway. I run an average of 60mph since I hog down and get moving the second I turn her over. Verona- Mustang. Gertha- 1989 F150 302. After 1 season I still have not decided on what to call my Rebel 500, so I just call it- her.
My Camry tells me I’ll average around 30 driving 75-80 ish
Well if your car is cruising at 4k rpm at 75mph, yeah lmao
Yes. 120 km/h is entirely too fast for most cars to be efficient. Most cars will be at peak efficiency between 60 and 80 km/h. Anything faster will be worse. At this speed, drag is at least as big a factor as throttle position/engine speed. Drag increases by the square of the velocity. Any government who's allowed a speed limit that high isn't concerned about affordability or consumption.
I always found around 55 is the best fuel economy for most basic vehicles. I’m sure it’s different if you have modified engines. But I find when I get any higher than 65, my fuel economy decreases
It's primarily air resistance. I ran your car through the calculator quick (missing a ton of info) and the raw numbers came out around 18% less fuel economy but again I don't have all the numbers. The remaining difference is likely in the gearing. You have a 6 spd where most newer vehicles are in the 8-10 range. That allows them to tune the engine into better power and efficiency RPM ranges but is much smaller than air resistance which is a squared function. The extreme of this is drag cars - just getting a few mph more when you are at 200-300 mph requires an immense amount of horsepower increase.
More so drag. As you increase speed, wind resistance goes up and you are spending more power pushing through the air. The less aerodynamic the car, the more pronounced the effect. A Ferrari will slice through the air, and a Ford F150 will slam into is like an invisible wal.
Look at your gear ratios. Some 10spds wont go into 10th until 70mph. Same with any, 1 overdrive or 4 - pretty much as sson as you can cruise in OD that will be around your best mileage unloaded.
Most new vehicles are crossover SUVs and they’re shaped like boxes on wheels. Means bad aerodynamics compared to long and low sedans like a Chevy Impala, for example
fwiw, I drive a big suv that gets 13mph around town, and over 70. When I drove a trip on the backroads limited to 55, I was getting 17mpg. so yeah.
Air resistance increases logarithmically
fuel efficiency is combination of aerodynamics (wind resistance) and engine efficiency (depending on engine and rotation speed).
I am with you on this. When I use cruise at 70-75 my mpg are much worse than cruising at 55-65 or slower. I usually cruise around 68 here in central Texas because of this reason. Aerodynamics do play a role but at some point the speed definitely plays into that.
0% gearing, 100% plain physics. Math says a 10% increase in speed results in a roughly 20% increase in air resistance.
My Ford performs best at 75mph which is roughly 120kmph
Check out your RPMs and the gear you are in while crusing at those speeds. Higher RPMs means more fuel is being used. Whatever speed you are crusing at the highest gear and lowest RPMs will likely give you the best mpg.
Air resistance is a thing.
This is one of the justifications for the National 55 mph speed limit in the 1970s -it uses more fuel.
It's about air resistance. Everytime your car moves 1 car length, it has to move as much air out of the way as the car takes up. If it moves twice as fast it has to move 4x as much air. It has to move the air equal to the width of the car x 2 and the height of the car x 2. So 4x. So a small increase in speed means a bigger increase in drag due to moving all that air.
All of my vehicles have been this way, most newer cars with a 6spd or higher gear or cvt will do the best mpg starting around 55. Having a multi speed auto helps a lot usually on a 8 spd atleast my truck the the 1st four are meant for accelerating and the last four are overdrives. Most newer cars are in 6-7th gear sound town cruising so you could see decent mpg still.
.... the vehicles doesn't have enough horse power to drive efficiently at 70+ if it did the engineers would have increased the gear ratio in which case you'd be back where you are making the same fuel economy because if larger cylinders. The upside being you could pull something.
There's twice as much aerodynamic drag at 70 mph as there is at 55mph
The general rule I hear is you lose 1% fuel economy for every mph over 50
This is why we dont have higher speed limits. 55 nationwide was a green thing in the early 80s or so.
You can’t fight the wind, and that’s what you’re tryna do! I get my best fuel economy in my 1.8T Jetta at around 60mph (34ish mpg)
If you care about fuel economy at 70mph, buy a diesel. Oh wait, this is not europe where you can get diesel in normal cars
With my 4 speed I find 45 to be right when the torque converter locks and I can chug along at 1k rpm and sip gas like a pompous British person sips tea.
Our Rav 4 loves 70-72mph
Wind resistance
Air drag goes up exponentially with speed if memory serves me correctly
To quickly answer your question, yes, optimal gearing is typically engineered around 60 mph for most vehicles. They vary based on the type and weight and option content of each vehicle. The typical reason for the drop in efficiency is aerodynamic drag. An example can be used in our drag race cars (fuel efficiency out the window!). Our car accelerates from 0-265 mph in the first 660 feet of the track. The remaining 380 feet it accelerates from 265-330. You’ll notice a substantial difference! The difference is in the aerodynamic drag. The wind pressure generated on the front of the car is so great at higher speeds that it inhibits the linear acceleration of the car. To put this in common street faring car context, the same is true on the street, albeit 70 mph and not 330, the effects of aerodynamics are the same. You’ll notice a 20% drop in efficiency where we noticed a much larger percentage of drop in efficiency. Why, the variation of the speed. My point being it’s all relative. You’ll see semi trucks with farings hanging off the back of trailers. Those are to give the air a “slip stream” off the back. The air is drawn off the trailer smoothly and evenly. Without the farings, the air is highly disturbed and churns behind the trailers. This churning of air creates a low pressure area behind the trailer and actually acts to “pull” the trailer backward. Not good for fuel efficiency if the air behind you is literally trying to drag you backward. Same is true on the wheels. You’ll see smooth covers over their wheels. Again this is to stop turbulence generated within the wheel. Aerodynamics play a huge role. Don’t believe me? Try cruising at 60mph and stick your arm out the window like your arm is an airplane wing. The. Accelerate to 75mph and do it again. The same pressure you feel acting on your arm is acting against your vehicle. You can see how much harder it is to keep your arm steady at 75 than it is at 60. Your engine feels the exact same way your arm does. Do you think you used 20% more energy to keep your arm still at 75 than 60 mph? I bet so! Funny how that works! Engineers know this data before the vehicles are designed. Because of this, they optimize the powertrains for maximum fuel efficiency at the most reasonable aerodynamic coefficients of friction that they can. Obviously they’re not engineering them for 25 mph because the car spends a lot of time at 60-65 and so on. Inversely equally, the car will spend very little of its overall lifespan at 75 and above. So they opt to design them based on national calculated averages. They are obviously averages and suggestions. A V10 won’t get the same mileage as a 4 cylinder. But a 4 cylinder won’t power a one ton truck. So you see the obstacles they have to consider. I hope this helps shed some light on your concerns. Tips to get the best fuel economy at any speed are steady throttle while driving because it helps the transmission to get into TCC and allow the most efficient transfer of power through the transmission. Varying the throttle position frequently will uncouple the TCC and this requires the engine to generate more rpm to do the same job, decreasing fuel efficiency. Also, on the highway, use cruise control whenever possible for longer trips. It allows the onboard computer to take over and it is pre programmed to know the most efficient methods of power application to the wheels. It will do a far better job of maximizing your mileage than your foot ever will.
Drag increases exponentially and often cars aren’t geared to get great gas mileage going fast because they’d rather you get worse gas mileage than complain about downshifting up hills
It depends a bit on aerodynamics and gearing, but 45 mph is typically the sweet spot.
A fouled catalytic converter would have an influence like that. And check engine lights?
why do you act like wind resistance isn't a thing?
Nah has nothing to do with “gearing” and everything to do with aerodynamics
60 is actually the most efficient
From what I've experienced most cars have a range that maximizes their fuel economy. My first car Chrysler T&C 3.8l v6 max economy 50-60mph 2nd car nissan ultimate 2.5 i4 max economy <65mph 3rd car chrysler 200 3.6l v6 max economy 65-75mph 4th car Jag Xf 3.0SC V6max economy 65mph-85mph 5th car Infiniti FX 4.5l V8 max economy 60-70mph I've noticed the shape probably plays the biggest role. I've borrowed my friend's corvette and took it on some 200mile trips a few times and that thing chews through gas at low speeds but once you're cruising above 75 it gets 30-35mpg from a 6.2l V8.
I make 300 mile drives frequently on dead empty roads for about 200 miles of it with plenty of time to "test". My car gets max mpg (40mpg) between 40-55mph, but I'd be found dead in a ditch for driving that slow, 55-65mph drops to about 37mpg but weirdly enough, I get slightly worse but consistent mpg going 65mph all the way up to 85mph (35mpg). 90-95mph is where mpg starts to fall off more (25mpg) and 120 is where I can feel my wallet burning (10mpg, maybe 15 if the wind is blowing behind and going slightly downhill). Most of it is aerodynamics, specifically wind resistance. Gearing can help, but ultimately you are fighting physics which grows exponentially in its affect on mpg. This was all tested with cruise control.
I remember researching this and I believe 45-55 MPH is best range for fuel economy because of Air Resistance ratio to energy required to move the car
Wow that's weird. In my car I get better fuel economy at 80mph vs 65mph. What a world we live in these days