Wow - so ur going to try huh?!
Let me give u my thoughts on the matter:
First off i dont think the car should use its batteries to run till 40. I think that should be done using the conventional motor (the gas one).
Once ur cruising at say a speed of 50 and ur speed is constant, then the battery should kick in to keep u moving (....so maybe u do need to know how much power is required ! - )
ok - so we need to know how much power first - right..!
i hear potatoes can power a light bulb (heheh j/k)
Keep us updated and once ur putting it together i would definitely love to give u a hand!