[BrianG]

There isn't a gearing limit per se. It comes down to weight, drivetrain/rolling resistances, and battery power.

I was using the 10XL on 14 cells geared 14/40 on my ~10lb Revo with stock tranny locked into wide-ratio second gear. Motor temps were a little warm, but not too bad. I had sized the motor for 5s lipo but never got around to getting them since I had three 14-cell packs already. 5s would have let me run better gearing (probably run std or narrow ratio gearset in the tranny) while maintaining decent track speed (~35mph) and cooler motor.

And that is true; NiMH cells may have a hard time keeping up with the 7XL effectively, but again depends on weight. If it takes X power to move Y lbs, it only stands to reason it would take less power to run less weight. Still, NiMH works, the voltage just dips more than a properly sized lipo pack.