Revolution Gear & Axle GM 10-Bolt 8.5in/8.6in 3.73 Ratio Face Hobbed Ring & Pinion Set

3.73 is the ratio most GM half-ton owners pick when they start towing or step up to 33 inch tires, and it is the factory towing ratio on many Silverado, Sierra, Tahoe and Suburban builds. The Revolution Gear & Axle GM10-373DC is a new 3.73 ring and pinion for the GM 8.5 in. and 8.6 in. 10-bolt rear. Buyers are usually doing one of three things: replacing a noisy factory 3.73 set, deepening a 3.08 or 3.42 truck for a trailer, or getting back the pull lost to bigger tires.

What 3.73 gears change in a 10-bolt truck

With 3.73 gears the driveshaft turns 3.73 times per turn of the rear wheels. Against a 3.42 that is about 9 percent more multiplication; against a 3.08 it is about 21 percent. The tire math is the reason it is so popular: a 3.42 truck moved from 30 to 33 inch tires effectively drops to about 3.11, and 3.73s bring it back to roughly 3.39, close to the stock feel. Revolution cuts this set face hobbed (2-cut) and laps it, with a standard rotation 30-spline pinion, an 8.5 in. ring gear on 10 x 7/16 in. bolts, and 8620 steel.

Signs you need new gears or a deeper ratio

• Rear end whine that changes between acceleration and coasting at the same road speed
• Transmission hunting between gears on grades or with a trailer, and shift points that feel late after a tire upgrade
• Speedometer reading low since going to taller tires
• Clunk on takeoff, or chipped teeth and metal on the magnet at a fluid change

How to confirm the axle and ratio

Look for the RPO sticker in the glove box: GT4 means 3.73, GU6 is 3.42, GU4 is 3.08 and GT5 is 4.10. A G80 code means a factory locker is in the carrier. If the sticker is missing, the axle ratio and code guide shows the tooth-count method. Rule out wheel bearings and tires before buying gears: a bearing growl tends to change while weaving, a gear whine with throttle. The GM 10-bolt vs 12-bolt guide helps if you are not sure which axle you have.

Fitment

Revolution fits this set to the GM 8.5 in. and 8.6 in. 10-bolt, not to specific vehicles. That covers the 8.6 rear under most 1999-up GM half-ton trucks and SUVs and the older 8.5 used in GM cars, C/K pickups and Blazers, plus the 1977-1991 K-series 8.5 front axle. The carrier break is at 2.56/2.73, so any 2.73-and-up carrier accepts these gears. Not covered: the 8.2 in. 10-bolt, the Oldsmobile 8.5, the 7.5/7.625, the 9.5 and 9.76 12-bolts, and the 2010-2015 Camaro IRS 8.6. Match the ring gear size and cover bolt count, or send us your year and VIN. Compare with the 3.42 version if you are staying on stock tires, or step to the 4.10 set for 35s.

Install notes

• Replace both pinion bearings, the carrier bearings, the crush sleeve and seal. The GM 8.5 Timken master overhaul kit covers 8.5 axles; 1999-2008 8.6 owners should use the GM 8.6 master overhaul kit, and 2009-up 8.6s take a different pinion bearing kit.
• Set depth, preload and backlash to Revolution's 8.5-8.6 figures: 0.037 in. typical pinion shim, 14-19 in-lb on new pinion bearings, 0.004-0.007 in. backlash for this 2-cut set, 65 ft-lb ring gear bolts and 60 ft-lb caps. Check the pattern on drive and coast.
• Moving to 3.73 is a good time to add traction; a Grizzly locker for 30-spline 8.5 and 8.6 axles fits the same carrier series.
• Fill with 75W-90 GL-5 (add friction modifier for a clutch posi), break the gears in gently with cool-down stops, change the oil at 500 miles, then recalibrate the speedometer for the new ratio.

Is 3.73 the right ratio for 33 inch tires on a Silverado 1500?

It is the usual pick for a half-ton on 33s, especially if it tows. A 3.42 truck on 33s with 3.73s ends up close to its original stock-tire feel.

Do I need to reprogram the truck after installing 3.73s?

Yes, if the ratio changed. Update the axle ratio and tire size so the speedometer, shift points and ABS read correctly. A like-for-like 3.73 swap needs nothing.

Will my factory carrier accept these gears?

If the axle came with 2.73 or deeper gears, yes. Revolution lists the carrier break for this family at 2.56/2.73.

How do I know my truck has an 8.6 and not a 9.5 or 9.76 12-bolt?

Count the cover bolts. The 8.5 and 8.6 have 10; the 9.5 and 9.76 have 12. The RPO sticker and axle tag also identify the axle.

What backlash should face hobbed 3.73 gears have?

Revolution's chart lists 0.004-0.007 in. for 2-cut gears in the 8.5 and 8.6, tighter than the 0.006-0.010 in. used for 5-cut sets.