[aerohead]

Clearly, the road load horsepower requirement of any vehicle, at any velocity, will track along with velocity.
How efficiently any particular vehicle behaved at any given velocity would have to do with the specifics of its powertrain and BSFC map, or BCFC-e map, if an EV.
* AeroStealth's 2014 F-150 will return 22- mpg at a constant 62- mph.
* 32-mpg at a constant 35-mph.
* The F-150 Lightning will be approx. 77-mpg at 62-mph.
* 112- mpg at 35 mph.
* The 'sweet-spot' would always be a on 'case-specific' basis.