I was wondering what the difference is when you drop one tooth on the engine gear for example to go from a 13-38 to a 12-38 is it a significant change? And also how much does dropping one tooth on the axle gear change thins for example from 13-38 to 13-37 are these changes significant? Thanks for any help on this topic.

I don't remember the exact numbers but changing the axle gear one tooth changes the gear ratio about 0.07 on the ratio. A 13/39 is 3.0 and a 13/40 is 3.07. Changing the drive gear one tooth changes the gear ratio about 0.25. So a 12/39 is 3.25. It takes 3 teeth on the axle to equal 1 tooth on the motor. Use the axle for small changes in gearing and the engine gear for larger changes.

It changes your gear ratio quite a bit.... So if you where shifting 4th to 5th before, w/ the new gearing you would probably shift 3rd to 4th instead... The other thing is that dropping a tooth on the front gear gives you some more "dig." I can't seem to run a 12 front b/c it drops my RPMs to much and I can't come off the corners....

i.e. (Honda CR250)
Front Back 2nd 3rd 4th 5th
12 38 13.97 11.50 9.50 8.26
13 38 12.89 10.61 8.77 7.62

I don't see a way to attached a file anymore, so if you post your email I will send you a complete gear chart...

I don't see a way to attached a file anymore, so if you post your email I will send you a complete gear chart...

Nevermind...
I blogged it...

i.e.
http://www.microracing.com/forum/blog.php?b=8