2017 GMC Sierra 1500 Transmission & Axle Gear Ratios

Transmission gear ratios

The 2017 GMC Sierra 1500 was available with a few different transmission options, each offering a unique set of gear ratios to cater to various driving needs.

6L80 6-Speed Automatic Transmission

8L90 8-Speed Automatic Transmission

Available axle ratios

The 2017 GMC Sierra 1500 offered a range of axle ratios, allowing owners to optimize their truck for towing, fuel economy, or a balanced approach. These ratios are crucial in determining the final drive ratio and, consequently, the truck's performance characteristics.

Note: Availability may vary based on trim level, engine, and optional packages.

How to identify your axle ratio

Identifying your Sierra 1500's axle ratio is straightforward. The most reliable method is to check the RPO (Regular Production Option) sticker, typically located on the glove box door. Look for a three-digit code starting with "G" that corresponds to the axle ratio.

• GU4: 3.08:1 axle ratio
• GU5: 3.23:1 axle ratio
• GU6: 3.42:1 axle ratio
• GT4: 3.73:1 axle ratio

Alternatively, some rear axles may have a metal tag attached to one of the differential cover bolts, which can also indicate the ratio. For a definitive answer, especially if modifying the vehicle, a VIN decoder or a GM dealership can provide the factory build sheet.

Towing vs MPG impact

The choice of axle ratio significantly impacts your truck's towing capability and fuel economy. It's a trade-off that ultimately depends on your primary use for the vehicle.

A numerically lower axle ratio (e.g., 3.08:1) means the engine will operate at lower RPMs at a given speed. This generally leads to better fuel economy on the highway, as the engine doesn't have to work as hard. However, it will reduce the truck's mechanical advantage, translating to lower towing capacity and slower acceleration, especially when hauling heavy loads.

Conversely, a numerically higher axle ratio (e.g., 3.42:1 or 3.73:1) causes the engine to rev higher at the same speed. This provides a greater mechanical advantage, resulting in increased towing capacity and improved acceleration. The downside is that the higher engine speeds will typically lead to worse fuel economy, particularly during highway driving.

For those who frequently tow heavy trailers, a higher axle ratio is often preferred to ensure adequate pulling power and reduce strain on the drivetrain. For daily drivers with minimal towing needs, a lower ratio can offer significant fuel savings.

Common issues

While the 2017 GMC Sierra 1500 generally has robust powertrains, some common issues can arise with differentials and axle components over time.

• Ring and Pinion Whine: A persistent high-pitched whine that increases with vehicle speed can indicate wear or improper setup of the ring and pinion gears. This often becomes more noticeable during deceleration and can be a sign of excessive backlash or worn gears.
• Carrier Bearing Wear: The carrier bearings support the differential carrier within the axle housing. Worn bearings can manifest as a growling or humming noise, especially when turning or accelerating. If left unaddressed, it can lead to further damage within the differential.
• Limited-Slip Clutch Pack Wear: Trucks equipped with a limited-slip differential rely on clutch packs to distribute power between the rear wheels. Over time, these clutches can wear out, leading to a loss of limited-slip function. Symptoms include a clunking or chattering noise from the rear end, especially when making turns at low speeds. Servicing these clutch packs, often by replacing the friction material and using the correct limited-slip additive in the differential fluid, can resolve the issue.

Regular differential fluid changes with the correct fluid type and viscosity are crucial for preventing premature wear and extending the life of these components.

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