This is a common misconception. Any electric generation from the car's motion while powered is going to take away from propelling the car. The loss of efficiency in the motor and generator make this a losing proposition. Even if you could design a 100% efficient motor and a 100% efficient generator, the best this arrangement could do is break even with no added charge. This applies to wind power generators as well since the drag they create makes the car harder to move for the motor.

Where this theory DOES apply is in braking. The average car releases braking energy as heat. Cars that have regenerative braking are recovering that energy when slowing or stopping by using the motor as a generator. Regenerative braking may add 10% or so to an electric car's overall range.

Hope that helps.