[Rivermaxx]

Although not always true, a rule of thumb is:
Hot motor, cool controller, cool batteries = undergeared (either increase pinion tooth count or decrease spur tooth count)
Cool motor, hot controller, hot batteries = overgeared (decrease pinion tooth count or increase spur tooth count)
Warm motor, warm controller, warm batteries = happy components
hot motor, hot ESC, and hot batteries = overgeared

Hot, warm, and cool vary as well, so here's another general rule:
Hot for a motor = generally over 160F (71C)
Hot for a controller = generally over 150F (66C)(or "thermal" range for the controller)
Hot for batteries = generally over 150F (66C) for nimh, 130F (54C) for lipo
Warm for a motor = generally 100F (38C) through 150F (66C)
Warm for a controller = generally 100F (38C) through 130F (54C)
Warm for batteries = generally 100F (38C) through 130F (54C) for nimh, ambient temp through 120F (49C) for lipo
Cool for all components = generally ambient temp and up to 10 degrees F (5.6C) more

The reasons the above are not always true are numerous, but may include:
Sticky drivetrain - will put un-necessary strain on electronics whether geared correctly or not.
Poor quality batteries - batteries will heat up regardless of gearing.
Wrong motor for the application - will put un-necessary strain on electronics whether geared correctly or not.
Wrong controller for the application - will put un-necessary strain on electronics whether geared correctly or not.

The list goes on, but this can give you an idea of things to watch for. btw I did not write this. The pinion needs to be bigger if you want to gear higher.