The radiator fan in a car currently is designed to spin either in response to a thermostat or continuously upon engine turnover. In either case, the power to turn the fan is provided by the car battery. If the fan were to be placed in front of the radiator (as opposed to the back as it currently is now), it could effectively harness the wind to turn it when the car is in motion. This 'pinwheel' could then be used as a generator to power another battery (the 'windmill battery') in the car.
When the car pulls up at a petrol (gasoline) station, the engine could be kept running and the windmill battery could be used to feed power into the grid using the car's ignition coil as a step up transformer to create a positive voltage gradient between the windmill battery and the power grid.
These 'millions of windmills' could obviate the need to build gigantic windmills. The vehicle operators/owners could be given credits on how much electricity they upload to the grid and use those credits toward a reduction in gasoline price.