RielTime Racing

http://www.robinsonracing.com/tips.html

Pick what size pitch size you are going to run, 48 or 64. Remember Tamiya pitch doesn't mesh well with standard 48 pitch, It's a bit different.

Then if you know what FDR (Final Drive Ratio) works for you and track you can figure out what size spur/pinion to use

Tip: Tamiya's 70t spur diameter size is the equvilant to a 93t 64 pitch diameter spur. The biggest Spur I've run in my TA05 is 104 64p anything really bigger is going to hang out the bottom of the chassis.

From what I understand a 13.5 brushless motor is supposed to be about the same as a 27t brushed stock motor. I've seen suggested FDR gear recommndations for 27t brushed/4-cell Vintage at: 5.4 - 5.7 FDR

Personally I've run FDR's at 8.0 - 5.0 in touring class depending on the track.

Hope this helps.