^^ Indeed, it's like a golf swing (I think, never actually played apart from PGA European Tour on my Mega Drive!), plenty of motion both before and after the point where you shoot is needed to get a good shot. And vertical movement is bad and automatically creates blur regardless of how smooth a pan is.
Probably not worth trying at this stage but manually focusing is more likely to get you a pin sharp image once you nail the panning technique. I always fire off a few shots at 1/1000 to find where the ideal focus point is, once they are sharp I move down to 1/100 or whatever. As long as the cars pass in roughly the same place it's fairly easy work from then on. Saying that though my auto-focus probably isn't up to the standard of the D-SLR boys here so it may be easier to just stick with AF on the better equipment.