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The gas mileage of my vehicle tends to improve the more I have been driving on the interstate on that tank of gas - if I go through a tank of gas without at any point driving on the interstate, I will typically get 24-26 MPG, but when I have driven almost exclusively on the interstate, I will typically get 26-29 MPG.

What confuses me is that, when traveling on the interstate, I am driving great distances at higher RPMs, which I would expect to correlate to more gas usage and thus worse gas mileage. Why does the inverse seem to be true?

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I have heard from my father that, mileage increases when you drive neither too fast nor too slow. – Immortal Player Jun 7 '14 at 1:40
It is not universally true. I drive a Prius. On the highway, I get 50 mpg while in the city, I get 55 mpg or more. – abby yorker Mar 8 '15 at 4:45

It is all about engine operating regime and how much braking and re-starting you do

  • The efficiency of internal combustion engines varies enormously over their range of safe operating conditions. In miles per gallon terms, of course, idling at rest is as bad as it could possible get. Manufactures take some trouble (in designing the whole powertrain) to ensure that the engine runs in relatively an efficient regime at highway speeds.
  • Accelerating the car takes more energy than just tooling along at a steady speed (that extra kinetic energy has to come from the fuel after all), but when you brake that energy is not recovered---it is converted to heat. So city driving with it's stops and starts means that you keep pouring energy into kinetic form and then promptly converting it to heat so that you have to go get some more from the fuel tank. Over and over again. That can't be good.

Try it with an electric or a good hybrid and your mileage will go down on the highway because it is dominated by air resistance.

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So, forgetting idling and accelerating in city driving, what makes highway driving at 70mph more efficient than driving on, for example, country roads at 45mph? Or is it? – user49034 Jun 7 '14 at 1:44
I don't know where you live, but where I grew up country roads involved more slowing down and speeding up than highways---where assuming no one got in my way, I could keep a steady speed for scores of miles at a time. It's the same problem with city driving, just on a smaller scale. – dmckee Jun 7 '14 at 1:47
Agreed, thanks for the elaboration. – user49034 Jun 7 '14 at 1:53
Direct illustration of this: Driving under cruise control is usually more fuel-efficient than driving with your foot on the gas, precisely because it maintains speed better. – keshlam Jun 7 '14 at 5:55

Gear ratios also play a role in the mileage. Typically, your gear ratios would be something like

$$ \begin{array}{c|c}\rm gear & \rm ratio \\ 1st & 3:1 \\ 2nd & 2:1 \\ 3rd & 1.5:1 \\4th & 1:1 \\ 5th & 0.75:1\end{array} $$

As your gear increases, which happens on interstates because you're traveling faster, it takes fewer rotations of your engine to turn your transmission (and thus wheels). At lower gears, i.e. city driving, your engine does quite a bit of work to turn the transmission.

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I've never been able to form a really clear argument about where the win comes from here, though I believe it is true. Care to elaborate? – dmckee Jun 7 '14 at 1:49
@dmckee: The higher the gear ratios, the higher the RPMs needed to achieve/maintain speed which means more fuel spent. – Kyle Kanos Jun 7 '14 at 2:20
That's the obvious part, but the losses to wind resistance are mounting as are the mechanical losses in the drive train. And just to confuse us more modern fuel injected systems can vary the air:fuel ratio as the engine loading changes. – dmckee Jun 7 '14 at 2:31
@dmckee: Rereading your comment, I now see what you meant (the missing d threw me off). I recall a PopSci article some years back suggesting that wind resistance starts dominating in the 55-65 mph range (at least for sedans) whereas 4th gear kicks in at about 40 mph. – Kyle Kanos Jun 7 '14 at 2:36

Dmckee is right, but I feel like I should emphasis this further: The constant stop/go traffic of city driving is brutal on fuel economy.

Gas/Diesel vehicles can't recuperate energy wasted when braking, unlike hybrids and electric vehicles (regenerative braking is one of their main advantages). You're constantly burning gas to accelerate, then wasting that energy as brake heat.

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Car engines don't have constant power output as a function of RPM. My car's manual states that the optimal power per fuel consumption of the engine is at ~3000 RPM, which corresponds roughly to 120 km/h on fifth gear. Apparently, the engines are made so that this is true for highway trips. Of course, such trips don't entail lots of acceleration and deceleration, which wastes a lot of energy. Check this link for details.

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