it depends on the size track/area you are running on. a 12/90 will top out quicker, and the motor will be running @ WOT longer, which can overheat the motor. overgearing and undergearing can both overheat the motor, I work up/down on pinions to find a "sweet spot", where the motor is plenty punchy, plenty fast, and doesnt have an overheating problem.
Your gearing also depends on what car you have it in....
--Joey --Venom Batteries & Chargers!!
Old School Racer & Vintage RC Car nut
RC10CE (X4),B2,B3,B4,B5, B5m, B6D - 10T,T2,T3,T4,T5M - B44.3
BRZ-444, RC10M, RC10L3 oval, L4 oval X 2