i would install a miata rear end with a low resistance generator instead of a center dif.. you could probably fit one larger enough that at highway speed it could protentially produce enough electricity to power the motor, taking the batteries out of the equation. you might need to add a few hp to the motor to make up for the added drag, but i think it would be worth it.
i am not sure of the currents / amps / all that stuff with ev cars though.
(if you take that idea and sell it, i want some money too)