I did an experiment on the interstate and drove at highway speeds to see if the mpg was comparable. It wasn't, so there's definitely another factor there. Here they cut groves in the concrete pavement on the interstate where as the highways are smooth asphalt. I've wondered if maybe rolling resistance is also a factor here. It was a pretty huge difference. Like more than 10% IIRC.
For EVs it's pretty simple, the faster you go, the more power you use, and the more air resistance, and the less miles / kWh. The more power you use, the more power is lost in wiring, the hotter the batteries get and less efficient they are, etc. There's probably some minimum efficient speed, but as you go faster, everything works against you to make things less efficient.
For gas engines it's quite a bit trickier, because a hotter engine gets more complete combustion and more power out the driveshaft per gallon (also more NOx, see VW) and there's a small range of RPMs where power output is best, combined with a transmission gear where efficiency is best, combined that gets you a target speed for best fuel to power at the wheels. But air resistance does get worse as you go faster. So you've got some things pushing you to go faster (sometimes much faster), and other things pushing you to go slower.
