You really want a fast shutter speed. Try 400 while panning. The motion of the car is not as fast as the motion of the wheels, so while the car will be sharp at the faster speed the wheels will still be blurred.

Panning a moving object at a medium-fast shutter speed will get results like this:


A slightly faster shutter speed or slower object will give this:


In either case, if the background had any texture, it would have been blurred.