[Finnster]

42K is not high per se, as said it would be fine in a plane app where your constant power pull is within specs. However we do use rpm as proxy for power, where the rule is (was) 28-32K was great for trackable levels of power in a 1/8th truggy or Revo. This yielded about 1500-1800W. Ie a 1Y on 4S, geared properly for ~40mph.

Take that same 1Y and spin to 42K, ie 6S. Geared the same that's more like 60mph. Amp draw is about same, but voltage increases by 50%. So does power then.
So you try to tone down the power/speed by gearing down, but now you are unloading the motor. I wish Lehner still had the graphs up, but they had very detailed data on load vs input/output power. Even for high end motors that reach 90%+ efficiency, at low loads, eff dropped to as bad as 50%, quite remarkably. Overloading the motor does not produce such drops.

So even if you are not producing that much power, the heat losses can still be terrible due to low eff. Try slow spd driving, sand (or snow) driving or driving w/ a loose slipper where the motor is lowly loaded. They will heat up the motor faster than high spd runs. You are far better off just dropping the cell count than the tooth count to get the desired speeds. If you want more runtime, use larger cells, not more of them (if you leave the motor the same.) Much has been made of the gearing, but the driving environment also is a factor. If you have the Revo all covered w/ snow, revving to the max and just kinda plowing thru snow drifts w/ big snowy rooster tails , then yea, that's a real challenge for any motor to stand up to.