So you’re not harvesting free energy; you’re paying for it with more fuel or battery.

Of course it's not free energy, but strictly theoretically if the turbine, say, doubles the air resistance in the area where it's installed and works at 80% efficiency, then instead of losing A amount of power you're losing 2A and getting 1.6A, for a total of 0.4A net loss. Now I have no idea if these numbers are even remotely realistic, but that's kind of beside the point. Of course the turbine would simply be turning battery into less battery, but that's better than the car's front turning battery into no battery.