2017 GMC Yukon Transmission & Axle Gear Ratios

Transmission gear ratios

The 2017 GMC Yukon offered a few transmission options depending on the engine and trim level. The most common was the robust 6L80 6-speed automatic, paired with the 5.3L V8. Denali models with the larger 6.2L V8 received the advanced 8L90 8-speed automatic transmission.

Hydra-Matic 6L80 (6-Speed Automatic)

Hydra-Matic 8L90 (8-Speed Automatic, Denali only)

Available axle ratios

The axle ratio selection significantly impacts the Yukon's performance characteristics, balancing between towing capability and fuel economy. Generally, a numerically higher ratio (e.g., 3.42) provides more torque to the wheels for better towing, while a numerically lower ratio (e.g., 3.08) results in lower engine RPMs at highway speeds, enhancing fuel efficiency.

How to identify your axle ratio

For your 2017 GMC Yukon, the easiest and most reliable way to identify your axle ratio is by checking the Service Parts Identification sticker (often called the RPO sticker). This sticker is typically located in the glove compartment, on the underside of the center console lid, or sometimes on the driver's side door jamb.

Look for a three-character RPO code, which designates the axle ratio. For the 2017 Yukon, the common codes are:

• GU4: Indicates a 3.08:1 axle ratio.
• GU6: Indicates a 3.42:1 axle ratio.

Unlike some other manufacturers, GM vehicles rarely use an axle tag for identification on modern vehicles. Therefore, relying on the RPO sticker is your best bet.

Towing vs MPG impact

Numerically lower ratio (e.g., 3.08):

• MPG: Generally results in better fuel economy, especially during highway driving. The engine operates at lower RPMs at a given speed.
• Towing: Provides less torque multiplication, meaning the engine has to work harder to move heavy loads. This can lead to increased engine strain and less efficient towing.

Numerically higher ratio (e.g., 3.42):

• Towing: Significantly improves towing capability. The increased torque multiplication allows the vehicle to pull heavier loads with less strain on the engine and transmission.
• MPG: Typically results in slightly reduced fuel economy, particularly at highway speeds, as the engine will be operating at higher RPMs.

Choosing the right axle ratio depends on your primary usage. If you frequently tow heavy trailers or carry significant loads, the 3.42 ratio is more suitable. For a daily driver concerned with maximizing fuel efficiency and only occasionally towing light loads, the 3.08 ratio would be preferable.

Common issues

While the 2017 GMC Yukon powertrains are generally reliable, a few common issues related to the differential and axles can arise over time:

• Ring & Pinion Whine: A persistent whining noise that changes with vehicle speed and often pitch during acceleration or deceleration. This can indicate worn or improperly set ring and pinion gears. This is more common in vehicles that have been heavily loaded, towed frequently, or if the differential fluid has not been maintained.
• Carrier Bearing Wear: The bearings supporting the differential carrier can wear out, leading to a clunking, humming, or growling noise, especially when accelerating from a stop or around corners. This wear can be accelerated by heavy towing or off-road use.
• Limited-Slip Clutch Wear (if equipped): Some Yukon models came with a limited-slip differential. Over time, the clutch packs within these differentials can wear out, leading to a chattering or shuddering sensation when turning, particularly at low speeds. Regular fluid changes with the correct friction modifier are crucial for preventing this.

Proper maintenance, including regular differential fluid changes with the correct type and viscosity of fluid, can significantly extend the lifespan of these components.

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