This depends on how much you can afford to spend and what camera you own, I would suggest one of the new telephoto superzooms, the Tamron or Sigma 150-600 mm lenses are ideal for motorsport, if you have a Nikon body you can buy the new 200-500 mm, I have ordered one of these having read good reviews. Second hand there are the Sigma 50-500mm and 150-500mm lenses, the first is supposidly the best of the two. For Canon the 100-400 is well regarded, now in version 2 so plenty of used version 1 examples around. Reach is needed for motorsport and I would suggest at least a 70-300mm lens if you think that the superzooms are out of your reach. With the longer lenses it is also useful to mount them on a monopod, I also use a ballhead set loose to allow similar panning movement to what I would have with a shorter lens hand held.