2015 GMC Sierra 2500HD: Transmission & Axle Gear Ratios

Transmission gear ratios

6-Speed Automatic (6L90, RPO MYD)

This heavy-duty 6-speed automatic transmission was standard on models with the 6.0L V8 gasoline engine.

Allison 1000 6-Speed Automatic (RPO MW7)

This robust 6-speed automatic transmission was paired with the 6.6L Duramax V8 turbo-diesel engine.

Available axle ratios

How to identify your axle ratio

For a 2015 GMC Sierra 2500HD, the easiest way to identify your axle ratio is by checking the Service Parts Identification (RPO) sticker. This sticker is located in the glove box. Look for one of the following RPO codes to determine your axle ratio:

• GT4: 3.73
• GT5: 4.10

For other manufacturers, the axle ratio information might be found on a tag bolted to the differential cover (Ford/Ram) or by using an online VIN decoder.

Towing vs MPG impact

Axle ratios can be a bit confusing. A numerically higher axle ratio (like 4.10) is often referred to as a "shorter" gear. This provides more torque to the wheels, which is ideal for towing heavy loads. However, the engine will run at a higher RPM on the highway, which can negatively impact fuel economy.

A numerically lower axle ratio (like 3.73) is often called a "taller" gear. This provides less torque to the wheels but allows the engine to run at a lower RPM on the highway. This is better for fuel economy but may not be ideal for maximum towing capacity.

For the 2015 GMC Sierra 2500HD, the 4.10 axle ratio is the preferred choice for those who frequently tow heavy trailers. The 3.73 axle ratio offers a good balance of towing capability and fuel efficiency, making it a great all-around choice.

Common issues

While the driveline in the 2015 GMC Sierra 2500HD is generally robust, some common issues can arise over time:

• Ring & Pinion Whine: A whining noise from the rear differential, particularly at certain speeds, can indicate a problem with the ring and pinion gears. This can be caused by improper setup, wear, or lack of lubrication.
• Carrier Bearing Failure: The carrier bearings support the differential carrier within the axle housing. If these bearings fail, you may hear a rumbling or growling noise from the differential.
• Limited-Slip Clutch Wear: On models equipped with a limited-slip differential (G80), the clutch packs can wear out over time. This will reduce the effectiveness of the limited-slip functionality, resulting in a one-wheel peel under acceleration.

If you are experiencing any of these issues, it is important to have your vehicle inspected by a qualified technician.

Find a driveline shop on Repairius.