Thanks, sir. You helped a lot.
As you can't get something for nothing you would cause the car to use more gas in proportion to the amount of electricity you produced (less losses).
There are some trials using pneumatic tubes across the road when a car passes over a puff or air drives a turbine to generate electricity - The idea is to power roadside signs and lighting. BUT it still causes the cars to use more gas.
A supermarket in the UK has a tilting ramp, as cars pass over it they generate electricity to power the car park machinery. - they use gas to get up the rap of course.
Experiments were done at Kingscross rail station in the UK to generate power from the many feet that pass through - special paving stones were laid, there was some success.
The closest your going to get to something for nothing is solar power or using waste heat to produce electricity - Now in a car exhaust there is a LOT of waste heat.