USA Standard Ring & Pinion Gear Set For GM 12 Bolt Car in a 3.90 Ratio
The USA Standard Gear ZG GM12P-390 is a 3.90 ring and pinion for the GM 12-bolt passenger car rear end, the 8.875 in. axle under 1964-1972 Chevelles, Camaros, Novas, Impalas, El Caminos, Monte Carlos, Firebirds and GTOs. People shopping for it usually have one of two problems: a classic muscle car rear that sings at cruise because the original gears are worn out, or a car with a tall 3.08 or 3.31 highway ratio that feels flat off the line with a big-block, a four-speed or sticky tires. A 3.90 is the classic street-strip ratio for these cars, and this set restores it with new gears made to exceed OEM specification.
The 12-bolt name comes from the twelve bolts that fasten the 8.875 in. ring gear to the carrier, not from the cover. This set has a 30 spline pinion that rides on two tapered bearings with preload set by a crush sleeve. The pinion turns 3.90 times for each turn of the axles (distributors often list the exact ratio as 3.91), so the engine reaches its power band sooner in every gear. The cost is cruise rpm: with a three-speed automatic or a four-speed with no overdrive, a 3.90 car spins well above 3,000 rpm at highway speed on period tire sizes, so think about how the car will be driven before choosing it over a 3.73.
The 12-bolt car rear has a carrier break between 3.90 and 4.10. This set is listed to fit the 3-series, 3.90-and-down carrier, so a car that already has 3.08, 3.31, 3.55, 3.73 or 3.90 gears keeps its case. If the car has 4.10 or deeper gears now, you need a 3-series case or a locker to match. Our carrier break explainer covers the details.
Many 10-bolt rears hide under the same cars, so identify the axle first. The car 12-bolt cover is an oval with a bulge at the top, and it differs from both the 10-bolt and the truck 12-bolt. Our 10-bolt vs 12-bolt differences guide walks through casting and cover identification, and the axle ratio guide shows how to read the axle code and count tire turns to confirm your current ratio. If the noise is your only complaint, separate gear noise from bearing noise with the accel vs decel whine diagnosis.
RANDYS lists this set for the GM 12-bolt passenger car rear in 1964-1972 Chevrolet Chevelle, Bel Air, Biscayne and Impala, 1967-1972 Camaro, 1964-1968 Chevy II, 1969-1972 Nova, 1965-1972 El Camino, 1966-1970 Caprice, 1970-1972 Monte Carlo, 1969-1972 Pontiac Firebird, 1970-1972 GTO and 1964-1970 Buick Wildcat. After decades of swaps, the axle under a given car may not be the one it was built with, so trust the cover and casting over the model badge, or send us your year, model and VIN. It does not fit the truck 12-bolt or the later 9.5 in. 12-bolt. Other axle gears and carriers are in our hard parts catalog.
Replace every bearing and the crush sleeve with the gears. The Yukon YK GM12P overhaul kit uses Timken bearings, and the USA Standard ZK GM12P kit is the lower-cost option. Yukon's setup chart for the 12-bolt passenger car lists a 0.030 in. starting pinion shim, 14-19 in-lb pinion bearing preload with new bearings, 0.006-0.011 in. backlash, 55 ft-lb ring gear bolts and 60 ft-lb bearing caps. Fill with 75W-90 gear oil plus friction modifier on Positraction cars, and drive gently through the ring and pinion break-in procedure before any hard launches. If you want traction too, the Dura Grip for 30 spline 12-bolt cars, 3.08-3.90 matches this carrier range.
| Brand | Yukon Gear & Axle |
| Part number | ZG GM12P-390 |
| Fits | USA Standard Ring & Pinion Gear Set For GM 12 Bolt Car in a 3.90 Ratio |
| Type | Final Drive Gears |
| Condition | New |
| Ships | Same business day (in stock), from the USA |
| Warranty | 1-year |
Doing this repair right usually means replacing these at the same time. Same transmission, ships together.
Not on the same carrier. This set is listed for the 3-series, 3.90-and-down carrier. A rear that has 4.10 or deeper gears uses the other carrier, so you would need a 3-series case or a matching locker.
No. It is for the 8.875 in. GM 12-bolt passenger car rear only. The truck 12-bolt, the 8.2 and 8.5 in. 10-bolts and the 9.5 in. 12-bolt all use different gear sets.
Without an overdrive transmission, a 3.90 car turns high rpm at highway speed. Many owners run it with a modern overdrive swap or taller tires. If the car spends most of its time on the highway with a non-overdrive transmission, a 3.55 or 3.73 is easier to live with.
Install a master overhaul kit with new pinion and carrier bearings, races, pinion seal, crush sleeve, shims and ring gear bolts. Inspect the Positraction clutches and the axle bearings and seals while the rear is apart.
Use 75W-90 gear oil, with friction modifier added when the rear has Positraction. Change it after the first few hundred break-in miles.
