2018 GMC Sierra 2500HD Transmission & Axle Gear Ratios

Transmission gear ratios

The 2018 GMC Sierra 2500HD was offered with two robust transmission options, each designed to handle the demands of a heavy-duty truck.

6-speed automatic (6L90)

This transmission was standard with the 6.0L Vortec V8 gasoline engine.

Allison 1000 6-speed automatic

Paired exclusively with the 6.6L Duramax V8 diesel engine, the Allison 1000 is renowned for its durability and smooth operation.

Available axle ratios

The 2018 GMC Sierra 2500HD offered a limited but effective range of axle ratios to complement its powertrains, primarily focusing on heavy-duty hauling and towing.

Note: While the 3.73 axle ratio was the most common across both engine options, the 4.10 ratio was specifically available with the gasoline engine for enhanced towing capability.

How to identify your axle ratio

For most GM trucks like the 2018 GMC Sierra 2500HD, the easiest way to identify your vehicle's axle ratio is by checking the Service Parts Identification sticker (often referred to as the RPO sticker). This sticker is typically located on the glove box door, or sometimes on the driver's side door jamb or under the hood. Look for a three-digit code that starts with "GT" or "HC."

• GT4: Indicates a 3.73:1 rear axle ratio.
• GT5: Indicates a 4.10:1 rear axle ratio.

Alternatively, you can contact a GM dealership with your VIN, and they can provide you with all the original build specifications for your truck.

Towing vs MPG impact

The choice of axle ratio significantly influences the balance between towing capability and fuel economy in your 2018 GMC Sierra 2500HD.

• Numerically lower ratios (e.g., 3.73): A numerically lower axle ratio means the engine turns fewer times for each rotation of the wheels. This generally results in better fuel economy during unloaded highway driving. However, it requires the engine to work harder when towing or hauling heavy loads, potentially leading to increased stress and reduced performance in demanding situations.

• Numerically higher ratios (e.g., 4.10): A numerically higher axle ratio means the engine turns more times for each rotation of the wheels. This provides more torque multiplication at the wheels, making it easier to launch and pull heavy loads. This is ideal for frequent towing or hauling. The trade-off is generally lower fuel economy, especially at highway speeds, as the engine will be revving higher to maintain the same speed.

For the 2018 Sierra 2500HD, if your primary use involves heavy towing (e.g., large fifth-wheel campers, equipment trailers), the 4.10 ratio with the gasoline engine is a strong contender. If your truck spends more time unladen or with lighter loads, and you prioritize fuel efficiency, the 3.73 ratio is a more balanced choice, especially when paired with the diesel engine's ample torque.

Common issues

While the powertrains in the Sierra 2500HD are robust, some common issues can arise with the driveline components over time, particularly if maintenance is neglected or the truck is consistently pushed to its limits.

• Ring and Pinion Whine: A common symptom of worn or improperly set up differential gears. This can manifest as a howling or whining sound that changes with vehicle speed and acceleration/deceleration.

• Carrier Bearing Wear: The carrier bearings support the differential carrier within the axle housing. Wear in these bearings can lead to noise (rumbling, grinding), vibrations, and eventually excessive play in the differential.

• Limited-Slip Clutch Wear: Many Sierra 2500HD models came with a limited-slip differential. Over time, the clutch packs within these differentials can wear out, leading to a loss of their intended function (improved traction) and sometimes shuddering or chattering during turns. Regular differential fluid changes with the correct friction modifier are crucial for longevity.

Find a driveline shop on Repairius.