The extra drag created by the wind generator on the vehicle would increase the fuel consumption beyond any energy that you would capture from the generator. It is likely that a generator operated by fuel would be much more efficient than a wind generator strapped to a moving vehicle powered by fuel. There are more energy transformations, and hence losses, in the fuel-wind-generator-batteries conversion than in the fuel-generator-batteries conversion. This assumes that the ambient wind encountered while traveling averages to zero because of shifting winds and changes in vehicle direction. If you are travelling into steady wind continuosly, e.g. while driving across Wyoming, then you might come out ahead with a wind generator mounted to the vehicle.