Ed Huntress wrote:

All else being equal, light cars typically get their best mileage at just
over 50 mph, and heavy ones somewhere between 45 and 50 mph. There are many
factors involved, including engine design.

At least you need to be able to run in the highest gear, or you lose.

Engine design obvously is key here. Air resistance scales with the
square of the speed; rolling resistance is a constant. Engine will
probably be square law as well on the resistance side.

Things like pumping loss and efficiency of unfilled cyclinders change
this; diesel behaves very different from gasoline for that reason.

In any case, even getting peak efficiency at 50 mph tells us not that the
car has high efficiency at higher speeds, but that the car has poor
efficiency at lower speeds, mostly for the reasons we've discussed.

True; but if you can't buy a better car, it is useful to know what speed
to preferably run it at.


Thomas