hierog,
Unfortunately, there is no "magic" number. It can only be measured on the track by way of laptimes, and battery runtime. Gearing the motor higher to run faster doesn't necessarily guarantee that you car will be faster all the way around the track If you overgear, the car may be sluggish out the turns, hurting your overall laptimes.
If you're looking for more torque, then going down to the next lowest ratio would be the logical choice to make. You will get a little more torque, but the bottom line is, are you faster around the track? If not, then you need to go back to the gear you last used. If you are, then you might want to try going a bit lower to see if you can go faster still. The goal is to find the gear combination that will give you the fastest lap times, assuming that your suspension setup is optimal (but that's another story).
