pinion size help please

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Hi, im building up a Tamiya 416x to race but i dont know what size pinion to use .. its got a 105 gear and im using a 13.5 novak motor.

someone suggested a 38 pinion and that would make the overall gear ratio about 5.68 they said.

Let me know if you use a tamiya 416x and might be able to help.

Thanks in advance Dan