Reading other threads the wind turbine idea often comes up. And the aerodynamic drag will not compensate for the savings in electrical load and therefore FE.
BUT, most cars already have a wind turbine in front of them. The radiator fan if electric is also an electrical generator, as all motors are (just the efficiency changes).
So if one were to want to try the wind turbine in front of the car idea, you already got one in front of your car
A little bit of ducting to get more air through it would be helpful.
Unfortunately I do not have the time to try this idea out at the moment, but when time permits I will try to give it a shot.
Some one with a volt and amp meter that has the cables easilly accessible might give it a go.
Going completely alternatorless might become feasible for very low power consumption cars. And the electrical signal to trigger the fan could go to a double throw relay to put the fan back in to fan mode from generator mode.