Driven teeth divided by drive teeth = the ratio.
Count the teeth on the drive (input/pinion) and driven (output/ring) gears.
Ratio = driven teeth / drive teeth.
A higher ratio multiplies torque more but raises RPM at speed.
Divide the number of teeth on the driven gear by the number on the drive gear. For example, a 41-tooth ring and 11-tooth pinion give a 3.73 final-drive ratio.
In a differential, the pinion is the small input gear and the ring is the large driven gear. Their teeth ratio is the final-drive ratio.
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