Going too slow is definitely inefficient for non-hybrid cars and trucks, but the optimal speed for most vehicles is certainly not higher than 55 MPH.

Average fuel economy for cars and light trucks decreases 12% going from 50 to 60 MPH: https://www.energy.gov/eere/vehicles/fact-982-june-19-2017-s...

Heavy trucks are the exception, they are typically designed for an operating point of 65 MPH. However, there are huge losses above that speed. No reason for a heavy truck to be going 70 or 75 MPH.

The American Trucking Association actually endorses a nation 65 MPH speed limit for trucks, which they say would save 280 million gallons of diesel per year: https://www.fleetowner.com/emissions-efficiency/article/2166...

The Tesla Model 3's computer's range calculator gives a max efficiency at 22mph.

In my gasoline car, max efficiency is at 29 mph, or 35 if it's dark, cold and raining (energy use of lights, fan and wipers is minimised by driving faster so you need the lights for fewer hours to go the same distance)

Really? The graphs I have seen put the optimal ICE car speed at about 30-50 mph depending on car aerodynamics. For EVs it is probably even lower as drag dominate earlier.