GM 8.5 10-Bolt Ring and Pinion Gear Set, 3.73 Ratio

This is the USA Standard Gear ZG GM8.5-373 gear set: a new ring gear and matching pinion cut for a 3.73 axle ratio in the GM 8.5-inch 10-bolt rear differential. It ships as a matched pair with 12 new ring gear bolts in the box, which is what you want, because ring gear bolts are a one-time-use fastener and reusing stretched ones is how a ring gear walks off the case.

You buy a set like this for one of two reasons. Either the original gears are done - a howl on decel, a whine that changes with throttle, chipped or spalled teeth on the drive side, or metal in the diff cover - or you are regearing on purpose after going to a taller tire or a different transmission and the truck no longer pulls in the gear it used to. Going to 3.73 puts the engine back in its powerband and takes the load off a converter that has been slipping to make up the difference. On an automatic that has been running hot, that alone can drop pan temps.

Setup is the whole job. Measure and record pinion depth before you tear it down, press the new pinion bearing on, shim the pinion to depth, set carrier bearing preload with side shims, then set backlash and check it with a dial indicator on the ring gear teeth. Paint the teeth and read the pattern on both drive and coast sides before you close it up - a pattern that rides too far toward the toe or the heel will get noisy and then it will fail. Plan on a new pinion seal and crush sleeve, a new carrier bearing set, and RTV or a cover gasket. If you have never set a pattern, budget the time or hand it to someone who has; these gears are only as quiet as the setup behind them.

USA Standard Gear covers these sets with a one-year warranty against defects. The gear set is the ring and pinion only - bearings, shims, crush sleeve and seals are sold separately.