2019 GMC Sierra 2500HD: Transmission & Axle Gear Ratios

Transmission Gear Ratios

The 2019 GMC Sierra 2500HD was offered with two heavy-duty transmissions, both paired with specific engines to optimize performance for towing, hauling, and daily driving.

Allison 1000 6-Speed Automatic

This legendary transmission was exclusively paired with the 6.6L Duramax Turbo-Diesel V8 engine. Known for its robust construction and smooth power delivery, the Allison 1000 is a favorite among those who tow heavy loads.

GM Hydra-Matic 6L90 6-Speed Automatic

This transmission was the standard and only option for the 6.0L Vortec V8 gasoline engine. It is a proven and reliable workhorse designed for the high-torque output of the gas V8.

Available Axle Ratios

The axle ratio is the final piece of the gearing puzzle, multiplying the torque delivered by the engine and transmission before it reaches the rear wheels. Selecting the right axle ratio is a critical decision that balances towing capability with fuel efficiency.

How to Identify Your Axle Ratio

Knowing your truck's specific axle ratio is crucial for maintenance, repairs, or if you're considering modifications. For the 2019 GMC Sierra 2500HD, the most reliable method is using the RPO code sticker.

• Service Parts Identification (RPO) Sticker: Open the driver's side door and look for a silver sticker in the door jamb or on the inside of the glove box door. This sticker contains a long list of three-character codes, which are the Regular Production Options for your specific truck. Scan the list for one of the codes in the table above: GT4 (3.73) or GT5 (4.10).
• VIN Decoder: While less direct, many online VIN decoder services can provide a build sheet for your truck, which will list the factory-installed axle ratio.
• Axle Tag (Less Common on GM): While more common on Ford and Ram trucks, some axles may have a small metal tag attached to one of the differential cover bolts. This tag is often stamped with the ratio (e.g., "3 73"). However, relying on the RPO sticker is the definitive method for GM vehicles.

Towing vs. MPG Impact

The relationship between axle ratios and performance is a classic trade-off between power and efficiency.

• Numerically Higher Ratio (e.g., 4.10): This is often called a "shorter" gear. It provides more torque to the wheels, which is ideal for towing heavy loads and better acceleration from a standstill. The engine will rev higher at any given speed, which increases fuel consumption and can lead to more engine noise on the highway. This is why the 4.10 ratio is paired with the gas engine to help it get heavy loads moving.

• Numerically Lower Ratio (e.g., 3.73): This is a "taller" gear. It reduces the final torque multiplication, resulting in lower engine RPMs at highway speeds. This translates directly to better fuel economy and a quieter ride during long-distance cruising. The immense low-end torque of the Duramax diesel engine allows it to use this more efficient ratio without sacrificing significant towing performance.

Common Issues

The driveline components in the 2019 Sierra 2500HD are built to be robust, but like any mechanical part, they can be subject to wear and tear, especially under heavy use.

• Ring & Pinion Whine: A whining or howling noise from the rear differential, particularly when accelerating or decelerating, can indicate excessive wear or improper spacing between the ring and pinion gears. This often requires a rebuild or replacement of the gear set by a specialist.
• Carrier Bearing Failure: The bearings that support the differential carrier can wear out over time, leading to a rumbling or grinding noise from the rear end that changes with vehicle speed.
• Limited-Slip Clutch Wear (if equipped): On models with the G80 automatic locking rear differential, the internal clutch packs can wear out. This might manifest as a chattering or binding feeling during low-speed turns, or a simple failure to lock when one wheel loses traction.

Find a driveline shop on Repairius.