My limited experience in this area would suggest that it would be a negative energy addition to the car.
There are a number of factors at play here - and a lot of variables - but I'll try and simplify as much as possible.
Assume that the wind direction is static, with a constant velocity, which would mean that a car (in theory) would spend as much time with a head wind as a tail wind.
In a static wind turbine, the wind passing over the blades is used to turn a generator - transferring energy from kinetic, to mechanical, to electrical. The amount of energy required to turn the blades is greater than the amount of mechanical energy at the turbine, which is also greater than the amount of electrical energy recovered. This is due to the fact that all conversions of energy are subject to some loss.
Transferring the turbine to a car, as is being suggested, means that the turbine is being moved against the air, as opposed to the air passing over the turbine. In this case there is a certain amount of mechanical energy required to driver the blades forward. The amount of energy 'generated' by the kinetic>mechanical>electrical process would be less than the amount of energy used by the electrical>mechanical process to drive the car forward.
In short, the extra drag outweighs the energy recovered.
The only potential application of this would be if the turbine was able to be deployed or withdrawn as necessary. E.g. when in a head wind the turbine may contribute additional energy to the car's systems, but in all other circumstances would be drawing energy from the car.
If you want to get a practical demonstration, it can be achieved with a pinwheel spinner like the one below.
Turn the 'blades' into the wind, and try moving it through the air at speed to turn the pinwheel. You can quickly feel that as the speed increases, the amount of drag generated is fairly substantial, before a turbine has even been added.
