2016 GMC Sierra 2500HD Transmission & Axle Gear Ratios

When outfitting or repairing a heavy-duty truck like the 2016 GMC Sierra 2500HD, understanding the transmission and axle gear ratios is crucial. The powertrain layout determines the truck's raw towing capability, highway cruising comfort, and fuel efficiency. Whether you are driving the 6.0L Vortec gas engine or the 6.6L Duramax turbo-diesel, knowing your exact drivetrain configuration can help you make informed decisions about modifications, repairs, and towing load limits.

Below is the complete reference guide for the 2016 GMC Sierra 2500HD transmission and axle ratios.

Transmission gear ratios

The 2016 GMC Sierra 2500HD features two robust six-speed automatic transmissions, each paired specifically to one of the engine options. The Hydra-Matic 6L90 handles the 6.0L V8 gas engine, while the legendary Allison 1000 is mated exclusively to the 6.6L Duramax diesel.

6L90 6-Speed Automatic (Standard with 6.0L V8 Gas)

The 6L90 transmission is designed with a deeply geared first gear to help get heavy loads moving off the line, combined with dual overdrive gears for highway fuel conservation.

Allison 1000 6-Speed Automatic (Available with 6.6L Duramax V8 Diesel)

The Allison 1000 transmission is built for extreme durability and heavy-duty towing. Its gear spread allows the massive torque of the Duramax engine to shine without needing exceptionally steep first gear ratios.

Available axle ratios

To simplify ordering and maximize efficiency based on the engine type, General Motors restricted the axle ratios for the 2016 GMC Sierra 2500HD to two specific options. Which one you have depends entirely on what engine sits under the hood.

How to identify your axle ratio

Identifying the rear axle ratio on your 2016 GMC Sierra 2500HD is incredibly straightforward thanks to General Motors' Regular Production Option (RPO) code system.

1. Locate the Service Parts Identification (SPID) sticker: On the 2016 Sierra, this sticker is typically located inside the glove box. In some rare configurations, it may be found on the driver’s side door jamb.
2. Search for the GT codes: Look through the alphabetical list of three-character codes.
   • If you see GT4, your truck is equipped with the 3.73 axle ratio.
   • If you see GT5, your truck has the 4.10 axle ratio.
   • If you see G80, your truck is also equipped with the Eaton heavy-duty automatic locking rear differential.
3. VIN Decoder: If the sticker is faded, damaged, or missing, you can enter your Vehicle Identification Number (VIN) into a reputable GM VIN decoder online, or call a local GMC dealership parts department. They can pull up the original build sheet for your exact truck.

Towing vs MPG impact

The relationship between your axle ratio, towing capacity, and fuel economy is a delicate balancing act.

A numerically lower ratio (such as a 3.31 or 3.55, though not factory options for the 2500HD) typically means better fuel economy. Lower ratios keep engine RPMs down at highway speeds, saving fuel and reducing engine noise, but they sacrifice low-end torque multiplication, making it harder to get heavy trailers moving.

A numerically higher ratio (like the 4.10 in the gas 2500HD) provides excellent mechanical torque multiplication. This is ideal for towing. It reduces strain on the engine and transmission when pulling heavily laden trailers up steep grades or taking off from a dead stop. The trade-off is that the engine must spin at higher RPMs at highway speeds, which negatively impacts your miles per gallon (MPG).

For the 2016 Sierra 2500HD, GM perfectly paired the 4.10 ratio with the gas engine to compensate for its lower torque compared to the diesel, while the Duramax diesel’s massive 765 lb-ft of torque allows it to effortlessly pull massive loads using the slightly more economical 3.73 ratio.

Common issues

While the 2500HD drivelines are exceptionally rugged, high-mileage or heavily worked trucks can develop some common rear differential and transmission issues:

• Ring & Pinion Whine: Consistent heavy towing, especially if the differential fluid is neglected, can cause the gear teeth on the ring and pinion to wear unevenly. This usually presents as a high-pitched whine that changes pitch depending on whether you are accelerating or coasting.
• G80 Locker Clunking: The Eaton "Gov-Loc" automatic locking differential (RPO G80) can sometimes become jerky or clunky as the internal flyweights and clutches wear out. If the fluid isn't changed at the correct intervals, or if the wrong fluid is used, the locker can fail to engage or become stuck.
• Carrier Bearing Wear: The heavy, two-piece driveshafts on long-bed or crew-cab models rely on a center support bearing (carrier bearing). Over time, the rubber isolating material degrades, causing a distinct shudder or vibration under acceleration from a stop.

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