We run a small technical track about 40'x60'. I gear my 13.5 at 108/39 (foam tires at 58mm) with the sintered rotor. The problem is even the 13.5 varies in gearing from motor to motor. A friend of mine is geared 84/29, my dad runs his at 104/36. Get the sintered rotor and gear it to the moon. The people who complain about it being to fast haven't run a well tuned co27.