If you want really nice panning shots, with the back ground uber blurred and the car nice and crisp then drop the shutter speed down to 1/160, maybe less when you get the hang of it..
Just makesure you follow the car even after you have taken the shot otherwise it will come out shit..lol
I just play around with the shutter speeds, have the rest set on Auto..
A drag strip sounds like a good place to practice panning tbh..
If you have Exif reader you will be able to see my settings on these pics..
Here's some of my panning results..