2022 GMC Sierra 2500HD Transmission & Axle Gear Ratios

The 2022 GMC Sierra 2500HD is a heavy-duty powerhouse designed for relentless towing, hauling, and off-road capability. A crucial factor in its exceptional performance is the pairing of its potent engines—the 6.6L Gas V8 and the 6.6L Duramax Turbo-Diesel—with highly specialized transmissions and rear axles. Understanding the gear ratios in your Sierra 2500HD is essential for maximizing its towing capacity and fuel efficiency.

Below is a comprehensive breakdown of the transmission internal gear ratios and rear axle ratios for the 2022 GMC Sierra 2500HD.

Transmission gear ratios

For the 2022 model year, the GMC Sierra 2500HD features two transmission options depending on your engine choice. The 6.6L Gas V8 is paired with the heavy-duty 6L90 6-speed automatic, while the 6.6L Duramax Diesel is mated to the legendary Allison 10-speed automatic (10L1000).

6-Speed Automatic Transmission (6L90)

Standard on 6.6L V8 Gas Engine

Allison 10-Speed Automatic Transmission (10L1000)

Standard on 6.6L Duramax Turbo-Diesel V8

Available axle ratios

To simplify powertrain configurations and ensure optimal performance for each engine's torque curve, GMC restricted the 2022 Sierra 2500HD to two primary axle ratios based on the chosen engine.

How to identify your axle ratio

Identifying the exact axle ratio on your 2022 GMC Sierra 2500HD is incredibly straightforward thanks to General Motors' standardized RPO (Regular Production Option) codes.

1. Locate the RPO Sticker: Open the driver's side door and check the door jamb, or check inside the glovebox door. You will see a silver or white sticker with a QR code and a grid of three-character alphanumeric codes.
2. Find the Axle Code: Scan the list (which is in alphabetical order) for the transmission/axle codes.
   • If you see GU6, your truck is equipped with the 3.42 axle ratio.
   • If you see GT4, your truck has the 3.73 axle ratio.
3. Check for a Locking Differential: While looking at the sticker, look for the code G80. If present, your truck is equipped with the Eaton heavy-duty automatic locking rear differential, a highly desirable feature for traction.
4. VIN Decoder: Alternatively, you can run your vehicle's 17-digit Vehicle Identification Number (VIN) through a GM VIN decoder or call a local dealership parts department to have them pull your build sheet.

Towing vs MPG impact

Gear ratios—both in the transmission and the rear differential—dictate the mechanical advantage the engine has over the wheels.

Numerically lower ratios (e.g., 3.42): Often referred to as "taller" gears, these allow the engine to spin at lower RPMs at highway speeds. This typically equates to better fuel economy (MPG). Because the 6.6L Duramax Diesel produces an earth-moving 910 lb-ft of torque at very low RPMs, it doesn't need a numerically high axle ratio to get heavy loads moving. The 3.42 ratio combined with the Allison 10-speed's ultra-low 4.54 first gear provides both brutal off-the-line pulling power and efficient highway cruising.

Numerically higher ratios (e.g., 3.73): Often called "shorter" gears, these multiply engine torque more effectively, resulting in better low-end acceleration and superior towing capability. Because the 6.6L Gas V8 makes its peak torque higher in the RPM range compared to the diesel, the 3.73 ratio is necessary to keep the engine in its optimal powerband when hauling heavy payloads or towing trailers up steep grades. The trade-off is a slight reduction in highway fuel economy, as the engine will cruise at higher RPMs.

Common issues

While the 2022 GMC Sierra 2500HD powertrain is generally robust and built to withstand severe duty, there are a few driveline considerations to keep in mind as the mileage creeps up:

• Ring & Pinion Whine: Heavy, continuous towing can accelerate wear on the differential fluid. If the fluid isn't changed at the severe-duty intervals, the ring and pinion gears can wear prematurely, leading to a noticeable whining noise that changes pitch with vehicle speed or acceleration/deceleration.
• G80 Locker Clunking: The Eaton automatic locking differential (G80) can sometimes exhibit a loud "clunk" when engaging. While some noise is normal due to its mechanical flyweight design, excessive banging or failure to disengage can indicate worn internal clutch packs or a broken governor mechanism.
• Carrier Bearing Wear: Models with a two-piece driveshaft may experience center support (carrier) bearing wear over time. Symptoms include a shudder or vibration under hard acceleration or when taking off with a heavy trailer.

Find a driveline shop on Repairius to keep your heavy-duty powertrain pulling strong for years to come.