Originally Posted by Johnny Wishbone

Got this from Paul Lemiux a while back, but still applies.

"Hi.
This is a rule of thumb for non timing speedos and any motor manufacture. depending on the track size and motor it could be a tooth or two on the pinion different. for timing speedos it depends completely on the speedo setting and their is many different gearing options based upon the timing settings on the speedo.
Everyone needs to take notice of motor temp at 3min when trying a new set up to ensure not to blow a motor. any manufactures motor will taper off in on track performance around 190deg (magnet gets weak) and most manufactures will not blow up untill 210-230 deg or more. if at 3 min your motor exceeds 170deg you are most likely too aggressive on gear or timing for a 6 min run.

Again this is for non timing speedos and just a good rule of thumb for a starting point.
21.5 turn, 3.6 overall
17.5 turn, 4.2 overall
13.5 turn, 5.1 overall
10.5 turn, 5.7 overall
5 turn, 8.1 overall
4.5 turn, 8.4 overall
4 turn, 8.9 overall

hope this helps a little. Thanks "