In the AE instructions it tells me 35 pinion with the 100 spur for A stock motor starting point. They guys at the local track are also saying that is what they use. It is an avrage size track. I may be getting misinfomred by the guys at the track. I have not cut the tires at this time nor will I until I get my feet wet racing again and get a feel for racing again. If I need to go to a 25 that is fine but why does the AE instructions tell us to start with that if it is hard to fit? Please keep your help coming. THANKS!!!!!!!