sunshark, this is a good idea to harvest the power from braking however if you want a constant power source then this idea won't work because you need speed up en maintain the speed of  the car first.. This always would take more power than the power you've gainded. 

you can easely reverse your electro engine if you decellerate with yor car into a power generator to charge charge up your batteries. Keep in mind that this energy is already taken from your power source.


my 2 cents 

Anything which relies on wind drag to make power will reduce the fuel economy. The air motion is being created by the forward motion of the vehicle, which is being driven by the engine.

Due to aerodynamic losses and losses in the wind generator's bearings and copper windings, the energy you recover from a 'wind drag spoiler' or anything like it will not exceed the energy it takes from the system to create it. You always get less, never more.