It all depends on the effect you want. If you want some blurring and a smear of a racecar going by, or the background a blur, use a slow shutter speed. If you want everything to be clear even in a quick pan, use a higher shutter speed. I personally would use a lower shutter speed in most cases just because I would follow the car around the track and I would want everything around it to look like a blur. There is no specific setting for this kind of thing. It all depends on the look you are trying to create. Play with it and see what you like.