With engines, they do this so that the engine is only operated near its most efficient state, which is high load over a moderate range of speeds. For an EV, I suggest a hard look at what the most efficient condition might be for each potential motor; I think you'll find that it again is under relatively high load and range of moderate speeds... but it's not going to be flat-out.Most of the cars basically throttle down hard until they hit a certain speed (30mph ish for best aerodynamic efficiency), and coast.
The efficiency of the battery is also dependent on the rate of discharge, meaning that you will need to understand the compromise between the better discharge efficiency of a larger battery (for a given power output) and the lower mass of a smaller battery. Or you can think of this as a compromise between the best operating point for the motor and the best for the battery.