Im running 75/25 with a 17.5 motor in my SC10 truck.
Im also running a Team Epic motor pretty much the same as a Revtech.

With a 13.5 motor Id run a 84 tooth spur start with a 20 tooth pinion
and gear for your track.
You might be able to run a 22 tooth with 78 spur.
Be sure to check temps thou after a 3-4 min run, if you cant put your hand
on the motor your overgeared.