It's generally the cars that come with the gear charts. And it's not that you can't run them way out of ratio, it's just that most drivers with most motors will find the best trade off between heat, acceleration, top speed, battery life etc with that ratio. A couple of teeth either side on pinion/spur won't hurt, depending on how you drive. But a huge change might.
FWIW i managed on my 2wd for a long time with (almost) stock set up, bang in the middle of the recommended ratio. Gave the car to a mate who is a *far* better driver, he cooked the motor (thermal cut out) as he was able to be "on power" for far more of the track than me. Added about 30 degrees to the hottest i'd ever seen.
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