Fuel Efficient Car

I would have to say that you have left out MANY important pieces to this puzzle. The question can not be answered with the very limited amount of information provided.

wot car

how heavy

how streamlined

wot batteries

is it hybrid

mechanism is far from simple.

how are you going to brake it ie stop it

STOP!!!!

From the questions you've asked you clearly do not know what you are doing. What you wish to do is a laudable goal but your understanding of the situation prevents you from even asking the right questions here. Modifying any machine that can kill people should never be done by somebody that does not have a firm grasp of the situation.

I am having trouble finding it, but there are companies who will install an electric motor in place of the driveshaft of a rear wheel drive vehicle. The attached device will allow the normal transmission to transfer power from the original engine through with additional horsepower added by the electric motor when needed to reduce fuel economy.

Some will also recharge through regenerative braking. If you really want to go nuts also replace the shock absorbers on your car with a regenerative type that accumulate pressure from the vehicles motion over bumps in the road which can be reused through various systems (I thought of powering my air conditioner with this one).

Drew