It really depends on what spur gear your running.
There is a guide in the manual that's quite helpful.
I'm running a SP 10.5 with a 76T spur and a 26T pinion, I'm just about to try out a 28T as im hitting top speed about halfway down the straight at my local track.
I think the guide say the recommended pinion size for a 76T spur is 24T to 32T.
I'm new to this so learning as I go alone, I keep getting told that as long as I can hold my thumb on the motor after a race im not running it too hot.
However im also told that a motor will run hot if it's under geared as well as over geared.
Hope that helps.
