The wind cars generate is not causally related to fossile fuel. Solar driven cars would also cause wind. And even now, generating additional energy from burning the same amount of fossile fuel can hardly be bad for the environment.
It isn't the same amount of fuel. If the car's engine is going to power something else besides the car, it is going to have to work harder.
Imagine the car is running in a vacuum and gets 30MPG. Now, add normal air resistance. It will not get 30MPG anymore. Now, add something else that further increases wind resistance. The car will get even fewer miles per gallon.
Wind power works by creating wind resistance. In order for there to be _less_ air resistance with them installed, they would have to be replacing something that caused even more wind resistance. But, that seems unlikely.
