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In rear-wheel drive automobiles the central drive shaft (or prop shaft) engages the differential through a hypoid gear(ring and pinion). The ring gear is mounted on the carrier of the planetary chain that forms the differential. This hypoid gear is a bevel gear that changes the direction of the drive rotation.
Referring to the top example above, the first letter in the code G denotes the ring gear diff size of the rear axle. In this case, it is an 8 ring gear. The second and third digit, both numbers 29 refer to the gear ratio.