2020 GMC Sierra 2500HD Transmission & Axle Gear Ratios

Transmission gear ratios

The 2020 GMC Sierra 2500HD was available with two primary transmission options, depending on the engine choice:

GM 6L90 6-Speed Automatic (paired with the 6.6L L8T gasoline engine)

Allison 10L1000 10-Speed Automatic (paired with the 6.6L Duramax L5P diesel engine)

Available axle ratios

The axle ratio plays a critical role in how your truck performs, influencing both towing capability and fuel economy. For the 2020 GMC Sierra 2500HD, the available axle ratios were primarily determined by the engine and transmission combination.

How to identify your axle ratio

Identifying your Sierra 2500HD's axle ratio is straightforward, thanks to GM's RPO (Regular Production Option) codes:

• RPO Sticker: The easiest way to find your axle ratio is by checking the RPO service parts identification sticker. This sticker is typically located in the glove box, on the driver's side door jamb, or occasionally under the hood. Look for codes like GT4 (3.73:1) or GT5 (3.31:1). The RPO code list on the sticker will contain a three-character code that designates your vehicle's specific axle ratio.
• VIN Decoder: If you don't have access to the physical sticker, a GM dealership can typically decode your VIN (Vehicle Identification Number) to provide you with all original factory specifications, including the axle ratio.
• Axle Tag (Less Common for newer GM): While more common on older vehicles or other manufacturers, some axles may have a metal tag bolted to one of the differential cover bolts with the ratio stamped on it. However, rely on the RPO sticker first for your 2020 Sierra.

Towing vs MPG impact

Understanding the relationship between axle ratio, towing, and fuel economy is crucial for heavy-duty truck owners:

• Numerically Lower Ratio (e.g., 3.31:1): A numerically lower axle ratio means the engine will turn fewer revolutions for every rotation of the wheels. This generally translates to better fuel economy on the highway, as the engine can cruise at lower RPMs. However, it also means the engine has to work harder from a standstill or when pulling heavy loads, potentially reducing towing capacity or making the truck feel less responsive under load.
• Numerically Higher Ratio (e.g., 3.73:1): A numerically higher axle ratio means the engine will turn more revolutions for every rotation of the wheels. This provides greater torque multiplication at the wheels, making it easier to accelerate, climb grades, and tow heavy trailers. The downside is that the engine will run at higher RPMs at any given speed, leading to decreased fuel economy, especially during highway driving.

For the 2020 Sierra 2500HD, the 3.73:1 ratio paired with the gasoline engine provides excellent all-around performance for a variety of tasks, while the 3.31:1 ratio with the diesel engine leverages the Duramax's substantial torque and the 10-speed transmission's wide gear spread to balance towing capability with efficiency.

Common issues

While the differentials in the 2020 GMC Sierra 2500HD are generally robust, some issues can arise over time or with heavy use:

• Ring and Pinion Whine: A high-pitched whine that increases with vehicle speed can indicate wear or improper setup of the ring and pinion gears. This often becomes noticeable during deceleration or at specific speed ranges.
• Carrier Bearing Wear: A rumbling or roaring noise, especially noticeable at higher speeds, could point to worn carrier bearings. If left unaddressed, this can lead to further differential damage.
• Limited-Slip Clutch Pack Wear: If equipped with a limited-slip differential, you might notice a chattering or grinding noise when turning tightly at low speeds. This is often a sign of worn clutch packs, which can reduce the effectiveness of the limited-slip feature.

Regular differential fluid changes with the correct type of fluid (including friction modifier if required for limited-slip units) are crucial for prolonging the life of these components.

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