Replaces a howling or chipped ring and pinion in the GM 8.5-inch 10-bolt rear axle, 3.73 ratio, with 12 new ring gear bolts included.
This page is for the owner of a Chevy or GMC half-ton, Tahoe, Blazer, Camaro or Monte Carlo with a 10-bolt rear that either sings on the highway or never recovered its pull after a tire upgrade. It is a USA Standard Gear ZG GM8.5-373: a new matched ring and pinion in a 3.73 ratio for the GM 8.5 and 8.6 inch 10-bolt, the most common rear axle under GM rear-drive trucks and many muscle cars.
Inside the 10-bolt, the pinion gear takes power from the driveshaft and turns the 8.5 inch ring gear bolted to the carrier. The tooth count ratio between them is the axle ratio, so with 3.73 gears the driveshaft spins 3.73 times per turn of the wheels. Compared with the 3.08, 3.23 or 3.42 gears GM used in many of these axles, 3.73 lets the engine sit higher in its torque curve at the same road speed. That is worth roughly two inches of tire height back: a truck that was geared 3.42 on 29 inch tires feels close to stock again with 3.73 on 31s, and a 3.08 truck on 33s gets most of its launch back. USA Standard sets are made to replace worn OEM gears at a fair price, and RANDYS lists this one with a 30 spline pinion and a 10 bolt ring gear.
Make sure the housing is an 8.5 or 8.6 and not a 7.5, 7.625 or 8.2 10-bolt; the smaller axles take different gears. The RPO code on the glove box sticker and the ring gear size settle it, and our 8.5 and 8.6 ring and pinion buyer's guide shows the visual differences. If the problem is noise, rule out wheel bearings and pinion bearings first; the GM 8.5 10-bolt problems guide covers how each one sounds. Unsure whether you have a 10-bolt or 12-bolt at all? See 10-bolt vs 12-bolt differences.
The ZG GM8.5-373 is listed for the GM 8.5 inch 10-bolt rear and the later 8.6 inch version. That covers many 1999-2017 Silverado and Sierra 1500 trucks, Tahoe, Yukon, Suburban 1500 and Avalanche, and on the 8.5 side 1970s-1990s C10/K10 and C/K1500 pickups, K5 Blazers and rear-drive cars such as Camaro, Firebird, Monte Carlo, Cutlass and Caprice. GM used several axles across those years, so not every vehicle on the list has an 8.5 or 8.6; check the RPO code or send us your year and VIN. If you would rather stay closer to stock, the Revolution 8.5/8.6 3.42 face hobbed set is the milder step. Browse more axle parts in our hard parts catalog.
Order bearings with the gears: the Timken GM 8.5 master bearing kit covers the bearing side of the job, and the Yukon 8.5 carrier installation kit is an option when only the carrier side needs attention. Use new ring gear bolts on the 10 bolt pattern. Revolution's 8.5-8.6 starting figures are a 0.037 inch pinion shim, 14-19 in-lb pinion bearing preload, 0.006-0.010 inch backlash (5-cut gears), 65 ft-lb ring gear bolts and 60 ft-lb caps. A carrier that ran 3.08, 3.23 or 3.42 gears normally accepts 3.73, but inspect the bearing journals and spider gears while it is out. A GM 8.6 rear cover kit is worth adding if your stamped cover is dented or leaking. Fill with 75W-90, plus friction modifier for a G80 or posi, drive easy for the first few hundred miles per our break-in procedure, and update the axle ratio in the computer on 1999-up trucks.
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Doing this repair right usually means replacing these at the same time. Same transmission, ships together.
Yes. This listing is the ZG GM8.5-373, a 3.73 ring and pinion for the GM 8.5 and 8.6 inch 10-bolt rear axle.
On a 1999-2017 half-ton that came with 3.08 or 3.42 gears, 3.73 suits a move to roughly 31-33 inch tires for daily driving. For 35s or heavy towing, most owners go deeper, to 4.10 or 4.56.
In most 8.5 and 8.6 axles, yes. A carrier that ran 3.08, 3.23 or 3.42 gears normally accepts 3.73. If your axle came with a very tall ratio, check the carrier break for your original gears before ordering.
No. Only the 8.5 and 8.6 inch 10-bolt. The 7.5, 7.625 and 8.2 inch axles use smaller gears with different dimensions.
A master bearing kit with pinion and carrier bearings, races, pinion seal and crush sleeve, new ring gear bolts, gear oil, and a dial indicator and inch-pound torque wrench to set backlash and preload.
On 1999 and newer GM trucks and SUVs, yes. The axle ratio is stored in the computer and affects the speedometer and shift points, so update it with a scan tool or tuner.
