Replacing the axle gears in a 3/4-ton GM pickup could be done in the driveway with jack stands, but for photography purposes we put the truck up on a two-post hoist.
Suspension lifts and big tires make a truck look good and more capable for off-roading, but these benefits come at a price. Tires with a larger diameter reduce the amount of leverage the engine has to move the truck down the road, and can force the engine to run in an rpm range that is less than optimal. Acceleration, fuel economy, speedometer errors, erratic transmission shift points, and added stress on the drivetrain are all problems that larger diameter tires can cause. The solution is to change the front gear ratios and rear differentials to compensate for the larger tires.
The '04 GMC 2500 Duramax truck in this step-by-step article is owned by Troy Sakota of Premier Performance, and is no stranger to aftermarket parts. The truck features a host of performance upgrades: an Edge Juice with an A2 monitor, an AFE intake, a MagnaFlow exhaust, a Fass fuel system, and Snow Performance water/methanol injection. The truck also has an 8-inch Fabtech lift and runs 37x12.50R22LT Toyo Open Country M/T tires on 22-inch Bozo wheels. After doing some calculations and talking to the guys at Yukon Gear, it was decided that 4.56 axle gears would work better than the factory 3.73 gears with 37-inch tires.
The 4.56 gears, bearings, and installation kits for the front AAM 925 and rear AAM 1150 axles came from Yukon Gear.
The rear axle in all Duramax-equipped pickups is an American Axle and Manufacturing (AAM) 1150. It features an 11 1/2-inch diameter ring gear and 30-spline 1 1/2-inch diameter axle shafts. This axle is almost identical to what's used in '03-and-newer Dodge 2500 and 3500 trucks. Dodge just adds an ABS tone ring to the differential. The independent front suspension (IFS) AAM 925 axle used in GM trucks is different however from the AAM 925 solid axle that Dodges use.
Jeremy Manning from Powerplay Performance in Ogden, Utah, was kind enough to let us use his shop and crew for the installation of the gears. Troy Sakota brought along Eric Brusman from Premier Performance to assist Powerplay Technician Justin Hustead with the installation.
| GEARING COMPARISON: 3.73 VS. 4.56 AXLE GEARS |
| Gear ratio | Stock 3.73 | 4.56 | Stock 3.73 | 4.56 |
| Speed (MPH) | 70 | 70 | 90 | 90 |
| Engine (RPM) | 1744 | 1853 | 1954 | 2352 |
| EGT (F) | 752 | 701 | 946 | 825 |
| Boost (PSI) | 4 | 3 | 11 | 6 |
| Engine Load | 35% | 26% | 47% | 31% |
The ResultsThe installation took the guys about five hours. This project required a lot of attention to detail, and it was a good thing the team documented where each part came from during the disassembly to make sure things went back together where they were supposed to.
With the new 4.56 gears in the axles, Sakota was eager to see the difference they would make to the truck's driving. After driving it with the new gears, he reported that the truck accelerated better, ran lower EGTs, the shift points were better, and he even claimed better fuel economy. From the data collected from his Edge A2 monitor during highway driving, it's clear that the 4.56 gears make the engine run in an rpm range where the truck is producing optimum horsepower and torque.