windmill on face of truck/bus ?
The windmill on the car doesnt work as your adding more force in the opposite direction to the work done by the engine. What about using the frictional energy on the face of bigger vehicles like trucks and buses that exist already to run a small windmill. Probably charge a battery or use it effectively in some other way.
I cannot see a direct negative to this except finding the way i can place a turbine without increasing the opposing force and using as much of the existing one