[MPaulHolmes]

I've been doing some figgerin. I guess the easiest way to find the energy usage of a car per mile at a given speed is the following:

Choose a flat stretch of road (no hill).
Choose a constant speed 's' (in miles per hour).
Measure the voltage 'V'.
Measure the current 'I'.

To find the energy usage per mile 'F' (in watt*hr/mi):

F = V*I/s


So, to find the energy usage per mile, just multiply the voltage by the current, and then divide by the speed (in miles per hour)!

By the way, what you are really measuring is how much force the batteries are pushing on the car to move it forward, because the units of watt-hours per mile can be converted to Newtons!
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Now, let's find the force fighting against the car moving forward. Let's assume we're in neutral, so the force we are about to find will be ONLY from the wind resistance and rolling resistance.

We must find the force 'F' that is resisting moving forward at a given speed.

Choose the speed 'v' (in meters per second).
Find out the mass of the car 'M' (in kilograms).

F = M*a, where 'a' is acceleration.

So, get up to a particular speed, and then put the car in neutral. Time it to see how long it takes the car to go from speed 'v' to speed v - dv, where dv is any amount you want, but the smaller the better. For example, let's say you are traveling at 30 meters/sec, and you time how long it takes to slow down to 25 meters/sec. Then dv = 5 meters in that case. Call the time measurement dt (in seconds).

Now, a = dv/dt, so

F = M*dv/dt. (the units are newtons)

To convert from newtons to watt*hr/mile, just multiply by 0.44704:

F = 0.44704*M*dv/dt (the units are now in watt*hr/mile)

The difference in this force measurement and the force from the batteries is the loss from the transmission, motor, controller, wires, etc.

You could also figure out the force from only rolling resistance by doing the above experiment while traveling very slowly, then you could figure out the drag coefficient of the car!