2017 GMC Sierra 2500HD Transmission & Axle Gear Ratios

Transmission gear ratios

The 2017 GMC Sierra 2500HD was offered with two primary transmission options: the 6-speed 6L90 automatic for gasoline engines and the 6-speed Allison 1000 automatic for diesel engines.

GM 6L90 Automatic Transmission (Gasoline)

Allison 1000 Automatic Transmission (Diesel)

Available axle ratios

The 2017 GMC Sierra 2500HD offered a limited but effective range of axle ratios tailored to its heavy-duty capabilities.

How to identify your axle ratio

For 2017 GMC Sierra 2500HD owners, the easiest and most reliable way to identify your vehicle's axle ratio is by checking the RPO (Regular Production Option) sticker. This sticker is typically located in the glove box on the inside of the door. Look for a three-character code. The code "GT4" indicates a 3.73:1 axle ratio. While some older vehicles may have an axle tag on the differential housing, the RPO sticker is the primary method for this model year.

Another option is to use a VIN decoder, which can often provide detailed build information, including axle ratios, based on your vehicle's unique VIN.

Towing vs MPG impact

The axle ratio plays a crucial role in balancing a truck's towing capability and fuel economy. A numerically higher axle ratio (e.g., 4.10:1, though not available as a distinct option on the 2017 2500HD beyond the standard 3.73) means the engine will turn more revolutions for every rotation of the wheels. This provides greater torque multiplication, resulting in enhanced towing power and quicker acceleration, but at the cost of higher engine RPMs at highway speeds, leading to decreased fuel efficiency.

Conversely, a numerically lower axle ratio (e.g., 3.42:1, again, not a direct option for the 2500HD in 2017 beyond the standard 3.73) would result in fewer engine revolutions per wheel rotation. This would typically improve fuel economy on the highway due to lower engine RPMs, but it would reduce the truck's pulling power and make it feel more sluggish when accelerating or towing heavy loads. The standard 3.73:1 ratio in the 2017 Sierra 2500HD is a well-engineered balance, providing ample towing capacity for typical heavy-duty applications while maintaining reasonable fuel economy for a truck in its class.

Common issues

Heavy-duty trucks like the Sierra 2500HD are built tough, but differentials and transmissions can experience wear over time. Here are some common issues to be aware of:

• Ring and Pinion Whine: A persistent whining noise that changes with vehicle speed, especially while accelerating or decelerating, can indicate wear or improper setup of the ring and pinion gears. This often necessitates replacement of the gear set.
• Carrier Bearing Wear: The carrier bearings support the differential carrier. If these bearings wear out, you might hear a rumble or howl, particularly when turning. Severe wear can lead to differential failure if not addressed.
• Limited-Slip Clutch Wear (if equipped): Trucks equipped with a limited-slip differential rely on clutch packs to transfer power to the wheel with the most traction. Over time, these clutches can wear out, leading to a loss of limited-slip function and potentially clunking or chattering noises during turns. Fluid changes with the correct friction modifier are crucial for their longevity.

Regular maintenance, including proper differential fluid changes at recommended intervals, is key to preventing these issues and ensuring the longevity of your drivetrain components.

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