[ajross]

I found a chart once where someone measured efficiency of some EV or another. And the rule of thumb was that the cars hit peak energy/distance efficiency at 40-45mph and drop to about half that at 90mph.

Which would amusingly put your 600 mile guess just about dead on.

[bell-cot]

I'd expect the Speed vs. MPG graphs for many conventional (gasoline or diesel engine) cars to be similar. Air resistance (of a given vehicle) is roughly proportional to the square of the velocity. And air resistance (or drag, to be more technical) doesn't care whether the car's wheels are being powered by batteries, burning fuel, or a wound-up giant spring.

[toast0]

All cars have to deal with the v squared air resistance, but EVs also have motors that get less efficient at higher speeds, and the more power you draw, the hotter your battery gets and that's a negative too. So really everything is aligned to punish you for going faster than a threshold speed.

With a convential vehicle, it's not so simple. A lot of things get worse, but engine efficiency is usually better with higher temperatures (which is why the VW TDI NOx emissions fix results in more CO2 emissions; NOx is a result of nitrogen in outside air being exposed to the hot engine, lowering the temperature reduces fuel efficiency and NOx production), and the gearing is designed to get maxium fuel efficiency at higher speeds. Of course, if it would be commercially acceptable, an engine and gearing could be designed to get max efficiency at a lower speed and have a 40 mph super efficient vehicle. Hybrids can do a lot better at running the engine near peak efficiency or having the engine off, allowing for higher mpg all over the speed spectrum.

[masklinn]

Air resistance starts very low though, until fairly high speeds other factors (e.g. rolling resistance) matter more. Weight is also relevant for climbing hills (more so for ICE which don’t reclaim any energy on the other side), …

[mcguire]

Rule of thumb: An gas or diesel vehicle will be most efficient at the lowest speed that it can travel in the highest gear. That's usually around 45.