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.

Quite possibly so for artificial tests where it's possible to maintain a
constant speed.

One key to low fuel consumption is the absence of braking and acceleration. It's
rare IME to be able to travel at those kinds of speeds without losing energy in
braking from time to time on typical roads. It's worse still at town speeds.

A consequence of this is that in the real world I actually see reduced fuel
comsumption ( compared to a lower average speed ) when I can drive 'freely' at
an average speed of around 60 mph, typically topping out at ~ 70 mph.

Graham