Dear 100 (Miles Per) Hour Board,
I drive to Layton occasionally and because of gas prices, I wonder every time whether it's more fuel efficient to drive at lower RPMs, which presumably uses less gas but means the engine runs longer (because it takes longer to get there), or at higher RPMs which uses more fuel but means the engine runs for a shorter time. Is there some kind of "supply and demand" type curve where one or the other is more efficient at different distances? How can I calculate this?
I was able to find a generalized answer for you at http://www.fueleconomy.gov . Keep in mind that every vehicle has a different optimum fuel economy (your owners manual may have more information). However, here is a graph to give you an idea. In general, "gas mileage usually decreases rapidly at speeds above 60 mph.
Now, for the above answer I assumed that when you asked about RPMs you were really talking about speed (since the vast majority of Americans drive cars with automatic transmissions). However, if you were drive a manual transmission and were asking about RPMs, here comes your answser. While driving an automatic you want to shift into a higher gear as soon as possible to save gas mileage. This will decrease acceleration and power but increase efficiency. However, don't shift too soon or you can cause unnecessary wear on your car. So in an automatic, driving at the same speed, it's gives better mileage to be in a higher gear which means lower RPMs.