Are you running it in your MF2? If so I'd suggest 20/88 as a starting point. I ran a 21T the day the last P2K2 dropped the magnets but gearing depends on the size of your track.
Go up one tooth at a time until your motor is too hot to comfortably hold your finger on for 3-5 seconds after a 5 min run. That's when you're too hot. Motor temp and performance depend also on how clean your motor comm and bushings are, wheel and tranny bearings spinning free etc.