[zelon88]

Efficiency is easy to ball park, but if you want to get precise then every vehicle is going to have it's own moving target. I would argue that no vehicle is especially efficient at 20mph for long durations. Sure you have no air resistance, but you also are spinning a 6 speed transmission when you really only need one. 90% of the car is being wasted. You might as well just drive a self propelled lawn mower.

Even two vehicles of the same make will be different due to differences in operating conditions.

An ICE engine, especially gasoline powered ones, operate with widely varying levels of efficiency. Lets think of a car in 4th gear at 50mph with 15% throttle applied. Now imagine climbing a steep hill. The car can stay in 4th gear and climb the hill with 65% throttle, or downshift to 3rd and climb the hill with 45% throttle. Speed alone doesn't determine fuel efficiency. Intake vacuum (or boost in certain applications) determines fuel efficiency. The trick is to try to go as fast as you can while consuming the least amount of fuel. There is nothing "efficient" about driving a 250hp, 3.0L V6 at 20mph.