Discus18-
It all depends. After your run wzs the motor too hot to touch? Are the Brushes burned up? How did the speed down the straight feel? How did the punch in the corners feel? There are a ton of variables to look at. I was running a 18 rollout on a carpet track with my 8x2. The motor was coming off the track pretty cool....I could hold it in my hands. I kept going up and up and up and now I am running a 22 rollout...which in my TC3 would have burned up the motor..so it all depends.
I know rollout is important (gear ratio)..but I am finding that if I can get more out of the motor and run it without going out of control or burning it up then I can do some insane gear ratios that some people would never think about doing. Try going up 1 pinion until the motor is too hot to handle or the brushes are burned up...you will know that is too high and go back down a tooth or two and you should be set.
Depending on the Track start with a 6.25-6.50Gear Ratio and go up or down depending on how the motor feels.
Mike Webb