I originally read this and completely missed the "behind the bumper" comment, and jumped straight to the idea of free energy. External wind turbine generators would cause too much drag, and you wouldn't put enough energy into the battery to recover what you lost from the drag.
However, I don't know how that would apply to inboard ones. The area in front of the radiator, for example, is already draggy, so do you lose less putting them there? Is it the same loses as externally placed WTGs?
For perspective, a standard onshore wind turbine can generate 2-4MW (depending on the model, obviously). I built a simulation of wind turbine control room a few years ago and there's a formula for calculating the power output of a turbine. (
here). Using a rotor diameter of 20cm (reasonable for a car?), wind speed of 50mph and a coefficiency of 0.4, I get an approximate power output of 129 watts.
But I've no idea how useful that would be in a car. Over a 100 mile drive would it generate enough to add additional miles? That's beyond my knowledge limit, so hopefully someone smarter can take this and convert it to car.