The wheels are body parts of the figure, and the standard Rotate dials ("xRot", "yRot", "zRot") are used to spin the wheels. The rotation center of a wheel body part (set in the Joint Editor) is the center of the wheel. Be sure to deactivate the "Bend" option of the wheels (in the "Info" window of the wheel body part). If the front of the car model is facing the front camera, the xRot dial spins the wheel, and the yRot dial can be used to steer.