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?
Answer
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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