Fuel efficiency at higher speeds

  1. Most cars even today are designed to have the best mileage at 55-65 miles per hour. Above that air and tire resistance begin to reduce the efficiency. Myth busters did a show and tests on this. It was shown to be true.

  2. It’s basically just a combination of drag/friction losses and static load. A car with a bigger engine will tend to have a higher peak efficient speed because it’s burning more fuel at idle and it you have to go faster to make that idle load worthwhile. Lower friction means that ideal speed is higher as well

  3. I really wish they would change this to 80 especially for hybrids. Of course I get great mileage around town but highway mileage is terrible

  4. The reason is because EPA highway tests are only at 60 mph. Car companies engineers their cars to be lost efficient on the tests, not real world.

  5. The best fuel efficiency typically will come at the lowest speed the car can maintain in top gear. Drag goes up exponentially with speed and will require more and more power to maintain speed.

  6. So, as a general rule for the post, cars with the smallest dimensions, and obviously the most aerodynamic, will be best at top speed because they're hitting less air. C7 might do better than virtually everything except maybe Miata or old 86. Even modern Minis are more like barn doors than the Corvettes. I'd love to see a test at like 80 mph to confirm.

  7. They were different cars. I’m more so asking in general though. Obviously, I understand that a car uses more fuel at higher speeds, but if I wanted to gauge a random car’s efficiency at 90 given that their highway mpg is, say, 30mpg, how do I know if it’s 24 or 18 at 90?

  8. Highway rating is at a much lower speed cycle, I think the average speed is like 50 mph or so. Interstate highway speeds are much higher and have even higher air resistance (resistance increases with the square of the speed; roughly speaking, even though 90 mph is 1.5x as fast as 60 mph, air resistance increases by 2.25x!).

  9. Gearing / rpm at whatever speed makes a big difference. I did a road trip in my '15 Grand Cherokee Limited V6 RWD. Did a 1000 mile segment in one day of driving (only stopped once for gas / lunch) and returned 29mpg despite driving 75mph. On the other hand, my work van (Transit Connect 2.5L 6 speed auto FWD) seems to only get 25-26mpg no matter if I drive it 55/65/75mph so go figure given it has two less cylinders (but also two less gears).

  10. Just based on the comparison you mentioned, the biggest difference there is the fact that you're comparing a smaller displacement turbocharged engine to a larger displacement naturally aspirated one. They may both be rated for 28 MPG on the highway based on government fuel economy tests, but once you go to much higher speeds, the smaller turbo engine will be less efficient. The main reason why smaller turbo engines are "more efficient" is because they are more efficient at lower speeds and loads. Once you demand more power from them, all of that is lost as the engine has to work much harder, which puts it outside of its efficiency range. Typically, gasoline engines are most efficient at around 1800 RPM and around 75% load. Keep in mind that this "load" is relative to displacement and not peak torque output, because a turbo engine under boost is at over 100% load. So at 90 MPH, that turbo engine is indeed working harder than the N/A V6, and thus getting worse efficiency.

  11. I just did a sustained 100mph/160kph run for around 150 kilometers, mileage was 7.5 litres / 100km or 31.3 US mpg. Not bad for nearly two tonne permanent AWD car (2015 Audi A7 3.0 TDI).

  12. Yeah, but it’s a diesel. A diesel burn is much more efficient than gasoline. If you had the gas equivalent, you probably wouldn’t see near 27 mpg

  13. Man I love TDI, I have 2.0 with 150 ps in a (european) Passat B8. Considering how my four cilinder is refined I cannot imagine how 3.0 feels like.

  14. This is highly inaccurate though. It assumes that EPA highway estimate is done exactly at 55mph but it's not. It contains cycles of different speeds with a maximum of 80.

  15. Interesting question. We know that mpg is basically going to come down to engine efficiency vs weight and aero where at city ratings weight is the main thing and aero means very little while the two eventually swap in importance as you go faster. But without knowing the gear ratios, drag, cross section, engine efficiency maps, etc. how can we guess at performance outside the test range?

  16. Road load measures the force needed to keep your vehicle cruising at any speed. The total force need to drive is road load + mass * acceleration. Your cars road load has three components to it, it is a second order polynomial: A + BVelocity + CVelocity2. The third component (C) is essentially and aerodynamic component. Since this component varies with the square of velocity the force needed to keep your vehicle cruising at higher speeds increases exponentially meaning your fuel economy decreases significantly. And that is not taking into account the extra force due to acceleration to even get to 90-95mph. Depending on your vehicles road load it is most efficient anywhere between 45-65 mph. Anything above that your mpg will significantly decrease. I've read papers saying that at 85 mph you're using 40% more fuel than at 70 mph. If you'd really want to research your cars efficiency you would need to know it's road loads and how steeply your cars road load increases as your velocity increases. But any manufacturer would not post this for obvious reasons. Your best bet is sticking around 70-75 mph to maximize your fuel economy on most highways in the US

  17. The biggest difference you’re going to find in high speed fuel efficiency is the tail form, that is where the greatest induced drag comes from, the tail vortex; and ‘Boat tail’ is the most efficient shape.

  18. There's no way to know, it has a lot to do with transmission and tire size. German cars can often cruise at high speeds (85+ mph) more economically because they are geared for Autobahn use I think

  19. I get 19 mpg average in my Aston Vantage. On freeway engine is doing 2k rpm at 88 mph. Onboard mpg indicated at 26 mpg average.

  20. Highway milage is at like 65 mph. If you want to get an idea of what the milage would be at 90-95 mph, I think the top speed is a good place to start. Most cars are limited, but it will at least give you an idea. Like, a 2019 Corvette has a top speed of around 190 mph So the extra 30 mph is a small effort from the car and just above half of what it can do. But you can contrast that with a 2022 Mitsubishi Mirage. It has a top speed of 102 mph. So 95 mph is about all it has to give. So while the Corvette might go from 25 mpg to 22-ish mpg. The Mirage might go from 43 mpg to 15-ish mpg.

  21. In general high performance cars overperform mpg estimates at high speeds. While turbocharged economy cars severely underperform

  22. The only difference I can think of between those two engines relevant to that would be the higher torque across the board of the v6.

  23. Hypothetically, if I were to cruise at 110+ for a while at a time, I, hypothetically, would notice that my mileage would still be pretty good, hypothetically around 25. It does take more to get there but it seems once there the engine is using the same amount of power for more distance.

  24. To receive the best mileage universally is 55 to 75. It seriously depends. Some cars state in their manual what's the most efficient speed for peak mileage. Some dont

  25. I've got an idea for a methodology but I need some vehicle specs, and some time. I need one of those bots to remind me about this.

  26. Are you sure you don’t mean km/h? Drag increases with the square of speed. There isn’t much if anything getting better than 20 MPH at 90 MPH.

  27. I recall somewhere that going faster than 70mph, every mph there after really starts going parabolic on how much fuel is used. Basically you're fighting the air more and more. Feel free to rip me apart on that.

  28. I weirdly got my best average mpg of 28 at about 85-90mph. I guess this thing just likes going fast.

  29. Final drive ratio and the coefficient of drag. The engine's power curve has an impact as well, but less so than the first two.

  30. gears, gear ratio, drag coefficient.. which all will let you estimate what RPM you will be at in teh highest gear. which is why things like corvettes do well in that area... low, slippery, and with lots of extra tall gears... can be in 7th at 1500rpm where something like a WRX would be in 6th at 3500rpm to hold 95mph

  31. I had always heard the peak economy was much lower, around 45-50 mph, with drag being the limiting factor. It could be higher now if aerodynamics have improved.

  32. My 07 Prius with 240xxx miles gets 38 mpg at 90-95mph. Took a big road trip from Indiana to our west and over multiple tanks this is what it got. While burning about 1qt / 1000 miles lol

  33. I’ve spent a good bit of time testing highway fuel efficiency over the years. Variables at speed include elevation changes, temperature, humidity, wind speed/direction, and most importantly, the driver’s technique and tactics.

  34. I’ve found that the correlation is simply torque at low rpm combined with long gearing. A larger displacement motor like an ls can chug at 2k rpm sustaining 80mph. Something with a turbo i4 needs to be at higher rpm to sustain that speed since fuel usage is largely dependent on TPS percentage. Ofc drag is a large variable but generally low rpm torque and long gearing is the formula, and both usually go hand in hand. A good example of bigish motor bad high speed fuel economy is an e92 m3 s65. Torque peak is high in the rev range. Gearing accounts for that too, so you get not so great fuel economy above 70 compared to similar sized v8s with low peak torque.

  35. Interesting question. But where do you live where you drive at 90-95mph consistently? That's 150km/h which is well into ticket territory where I live. Maybe on the German Autobahn but given you're using miles that's unlikely.

  36. That's not that unusual in the southern US. When I drive between Atlanta, Charlotte, and Raleigh I usually cruise at 80 mph, and I'm always getting passed. So a lot of the faster driving cars are probably going 90+ mph. When I visited Texas last year, I swear half the cars were going 100 mph lol.

  37. I think air drag becomes one of the heaviest factors, as well as gear ratios. So bigger cars will probably have a more noticeable impact on mileage with increased speed, compared to a smaller car where drag is less impactful

  38. Turbocharged engines will return lower efficiency when they go into boost as they will run a bit richer to cool the engine. When they go into boost will depend on the engine/turbocharger and gearing.

  39. From what I understand from just skimming over various topics and basic physics, the faster you go, the more drag you have to fight given that all other factors being equal. That drag vs speed, if plotted out on a graph would start to look exponencial if plotted out far enough. So, the faster you drive above a certain range, the drivetrain would have to expend more energy at an inefficient operating range in order to climb or maintain that higher speed, hence worse fuel economy.

  40. drag, gear ratios of the usual 65 mph efficiency or higher speed efficiency. also, an inline 4 turbo works harder to get to 80 than a v8 works to get to 110 mph.

  41. Air drag is the biggest factor by far, nothing else comes even close. And NO, air drag is not just the drag coefficient. It is the drag coefficient multiplied by the frontal area. That's why taller vehicles are much less efficient, bc of their bigger frontal area.

  42. The higher you are in vehicle speed relevent to RPM will determine the fuel required. More exhaust = more intake = more fuel.

  43. Turbo cars will get awful gas mileage at high speeds because turbos are a marketing gimmick. A turbo allows the manufacturer to claim a high peak HP number and a high MPG number - but of course you can only get one or the other, not both.

  44. Modern turbos can get good MPGs. My civic SI (1.5 liter turbo) will get 40+ on a road trip. They are just more flexible. You CAN get good mileage and good power from the same engine, just not at the same time.

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