[brian_]

Most of the cars basically throttle down hard until they hit a certain speed (30mph ish for best aerodynamic efficiency), and coast.

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.

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.

 

[madderscience]

below 30mph rolling resistance is probably the biggest factor, at least with a more or less standard car chassis. run high PSI narrow tires, and strip absolutely as much weigh as possible out of the car. run the minimum amount of lightest oil possible. you aren't going for longetivity of the mechanicals.

For drivetrain, the fewer moving pieces the better. You will need some kind of gear reduction but maybe only one ratio if you can do a slow start and then tool along at 30mph. pick gear ratio such that your motor runs near the peak of its efficiency curve. for most EV sized motors that is probably a few thousand RPM.

figure out how to make sure the brake pads fully retract from the drums/discs. maybe only run brakes on one axle. make sure the alignment is perfect.

for aerodynamics, strip all the trim, mirrors, etc. off the car, make fender skirts and a belly pan and all that stuff. lower the suspension as much as possible but run the largest diameter tires that fit. At lower speed all that won't do as much as it would on the freeway, but it will matter.

If you have an air cooled or TEFC motor, maybe take out the integral fan (or machine it off of the rotor). At 30mph you probalby won't be making enough horsepower to need much cooling as you sumrise.