If the car is stationary and pointed into the wind, then the wind turbine will generate power. Likewise if you are driving into a head-wind, you will have a net gain of energy. If you are driving with a tail-wind, you will lose energy.
Since we tend to drive in all directions, the overall energy from the car-mounted turbine will be negative (ie it is a loss). As was explained, the turbine pushes back on the car, so you need to use even more energy to move forward.
As suggested, a turbine fixed on your home/garage would be a better bet (larger blades then what you can fit on a car, more energy).
One interesting mind game - imagine a full-sized wind turbine mounted on the roof of your car. Some of the wind energy will move the car back, but since the blades are angled, some of that energy will produce a rotational torque (on the generator shaft) - this will tend to push the car sideways (you would probably have bad tire-wear). If you were driving into a strong wind, then hit a big bump in the road, you may land with the car on its side!
A bonus points math question - under what conditions could you hit a bump/ramp, get airborne, do a full rotation, and land back on your wheels?