If you mean a wind turbine to collect energy, that will create drag and slow down the car, or suck more fuel to maintain speed.

Yes the main purpose will be to fit a wind turbine to generate electricity. The difference is that to design one so that it will replace the function of a rear-wing on performance cars. Rear-wing on a car creates a down ward drag which enable the rear tiers to get a better grip and therefore makes the car go faster because there is less wasted energy. So the drag from the wind turbine will actually make the car go faster not slower.

What I want to say is that not all drag slows the car down if it helps drive wheel to maintain more traction at high speeds. e.g rear-wing.

There is an assumption that energy is wasted because of tire slippage. Under normal and not extreme variations of acceleration and deceleration tires do not slip and waste energy. The doodad wing at the rear of an ordinary car is in the class of the old tailfins which were pretty purely design decoration with little or no real function.

That wing on high performance cars actually increases the downward pressure on the tires the faster the car goes. In this way the car will behave as if it is heavier at higher speeds without the extra weight at lower speeds. The down pressure helps the tires stick to the road, increasing drag, which the car must overcome to go faster. You may have noticed that high performance cars are not exactly fuel efficient vehicles. That's why.

"not all drag slows the car down" ?? Yes it does. That's what 'drag' means. Yes, you can sacrifice a small amount of slowing-down to gain more traction, but all drag is a force resisting thrust.

Wings always slow a car or airplane for that matter. But the downforce increases cornering speed and safety for a car. Planes with smaller wings are faster, but harder to get into the air.

I've never seen a turbine used for a wing, either driving or driven. I think to get any (downward) lift-force, the design of the turbine would have to have a large annual wing incorporated--which is contrary to the need of the turbine to have a venturi shape. Probably not possible.

Would also inevitably add weight. Spoilers will always be lighter than a comparable turbine. Also some yet unknown loss from elastic forces in the tires. You can't designate any force for one explicit purpose.