[FrankJ]

Originally Posted by asid61

Correct me if I'm off, but shouldn't the pinion tooth load only be proportional to the mechanical power the motor is outputting and the pitch radius of the pinion, and be independent of the gear ratio?

Right concept, but wrong terms. Actual it is related to torque. A brushed DC motor at stall is transmitting maximum gear load but no power. The actual force transmitted is the same on both the big gear and small gear. The small gear wears faster because there are fewer of them. There is a practical limit on how small of a gear you can make where the geometry falls apart.