Yukon Gear High Performance Gear Set For GM 8.5in & 8.6in in a 4.88 Ratio

The 4.88 is the deep end of the GM 10-bolt range. People order the Yukon Gear & Axle YG GM8.5-488 when they have put 35 inch or taller tires on a Silverado, Sierra, Tahoe, K1500 or K5 Blazer and the truck now feels like it is starting in second gear, or when a Camaro, Nova or Chevelle drag car with a short tire needs hard launches. It is a new ring and pinion with an 8.50 in. ring gear, a 30-spline pinion and 10 ring gear bolts, built for both the 8.5 and the later 8.6.

What a 4.88 ring and pinion does in a GM 8.5 or 8.6

The pinion turns 4.88 times for each rotation of the axle shafts. Big tires act like a taller gear: going from a stock 30 inch tire to a 35 effectively raises the overall gearing by roughly a sixth, which drops the engine out of its power band, makes the transmission hunt between gears and heats the fluid. A 4.88 set pulls engine rpm back to where it was, restores off-the-line torque and gives back usable crawl in low range. In a light car with stock-diameter tires, the same ratio trades highway comfort for quick launches. To fit that many ring gear teeth on an 8.50 in. gear, the pinion head is smaller than on a 3.08 or 3.42 set, so careful setup and a gentle break-in matter more with this ratio.

Symptoms this ratio fixes

• Lazy acceleration, constant downshifts and high transmission temperatures after a move to 35 inch tires
• Speedometer reading low and torque converter lockup cycling on the highway with oversized tires
• Not enough crawl control in low range on rocks or steep trails
• Whine, clunk or chipped teeth from the original gears, if you are rebuilding the axle anyway

How to confirm 4.88 is the right ratio

Start with the ratio you have now (RPO code on the glove box label, or count the teeth) and the tire size you actually run. Our regear after bigger tires guide lays out typical ratios by tire size and transmission, and shows when 4.56 is enough and when 4.88 is worth it. Remember that a 4x4 needs the front and rear ratios to match exactly, so if this goes in the rear of a K-series truck or Blazer, the front must be regeared to 4.88 too. Our 8.5 / 8.6 ring and pinion buyer's guide helps confirm you have an 8.5 or 8.6 and not a 7.5, 7.625 or 8.2 10-bolt, which use different gears.

Fitment

Yukon's fitment table matches the other 8.5 and 8.6 sets: 1999-2017 Silverado 1500 and Sierra 1500, Tahoe, Yukon and Escalade through 2013-2017 depending on model, Suburban and Yukon XL 1500, Avalanche, Trailblazer, Envoy, Rainier, Colorado, Canyon, Hummer H3 and Express and Savana 1500 vans with the 8.6 rear; and the 8.5 rear in 1970s-1990s GM cars, C and K pickups, Suburbans, the K5 Blazer and G-series vans. Yukon also lists 8.5 front axles on 1977-1991 K-series trucks and Blazers, the axle you would regear to match a 4.88 rear, using the GM 8.5 front master overhaul kit for its bearings. It does not fit the 8.6 IRS, or the 9.5 and 9.76 12-bolts found in some half-tons. Housing and axle spline counts changed over the years, so match your axle, or send us your year and VIN before you order. Browse the rest of our differential rebuild kits for matching parts.

Install notes

Set the gears up with new bearings from a kit such as the GM 8.5 Timken master overhaul kit. Yukon's specifications for the 8.5 and 8.6 are 14-19 in-lb new pinion bearing preload, 0.006-0.011 in. backlash, 65 ft-lb ring gear bolts and 60 ft-lb bearing caps, starting from a 0.037 in. OEM pinion shim; final depth comes from the contact pattern. If your carrier is worn or you want a locker while it is apart, a complete loaded 30-spline open carrier is an option for 30-spline axles. Fill with 75W-90 GL-5 gear oil. Deep gears run hotter while new, so avoid towing and long highway runs for the first few hundred miles, let the axle cool when it gets hot, and change the oil at about 500 miles, as described in our ring and pinion break-in procedure. On computer-controlled trucks, program the new axle ratio and tire size so the speedometer, shift points and ABS read correctly.

What tire size goes with 4.88 gears in a GM half-ton?

4.88 is usually chosen for 35 to 37 inch tires on a truck or SUV, or for drag cars with stock-height tires. Your transmission, engine and how you drive also matter, so compare with the ratio chart in our regear guide.

Do I need to regear the front axle too?

Yes, on any 4x4. Front and rear ratios must match exactly or the drivetrain binds in 4WD and the transfer case can be damaged. Yukon lists this set for 8.5 front axles on 1977-1991 K-series trucks and Blazers.

Will a 4.88 fit my existing carrier?

GM 8.5 and 8.6 carriers are not split into ratio series the way some Dana axles are, so in most cases the ring gear bolts to the carrier you have. Inspect the carrier and spider gears while it is apart.

Do 4.88 gears need a special break-in?

Deep gears generate more heat while they seat. Drive gently for the first few hundred miles, avoid towing, let the axle cool if it gets hot, and change the gear oil at about 500 miles.

Will my speedometer be wrong after installing 4.88s?

If you change the ratio, yes. On electronically controlled trucks and SUVs, use a programmer to update the axle ratio and tire size.