ORIGINAL: 46u

Keep in mind I am running an OS 18TZ so the gearing I am running may not be good for your application as it if fairly high. I run a 15CB with a 49 spur and might go up even higher. I thing they make the Savage race CB from 13 to 20 teeth. Keep in mind you have to change your bearings to use this. For more information you can all ways go to HPI web site and look at the MT2 SS manual.
http://www2.towerhobbies.com/cgi-bin...?&I=LXLST7&P=Z
This is where I get my bearings, they are good bearings but inexpressive.
http://www.avidrc.com/