I am assuming your running 48P. What worked best for me was a 25/72 at our LHS, but having said that the track layout and motor temp made final gearing choices. Tight courses it ended up being 23/72 and motor temp at the end of 5 minutes was around 135-150 F.
I have since switched over to 64P in both my sons as well as my car. Right now my car is set up with 33/94 and is is 32/100. His motor temp is higher than mine by 10 degrees. Both motors have been "tweaked".