If you want to get some panning shots (sharp car, blurred backgrounds) you need to pan (follow the car) as you shoot. I have found around about 1/125 shutter speed works for me with a 200mm or so lens. I usually shoot on Tav (shutter priority), I start the day at around 1/200 or even 1/250 until I get the hang of panning again (keep checking every now and then how sharp the shots are) and as the day goes on bring the shutter speed down till I get to where I can no longer get sharp shots.
Hope this helps!