2019 GMC Sierra 1500: Transmission & Axle Gear Ratios

Transmission Gear Ratios

The 2019 GMC Sierra 1500 was available with three different automatic transmissions, each paired with specific engines in the lineup.

6-Speed Automatic (6L80)

This transmission was typically found on the base V6 models.

8-Speed Automatic (8L90)

The 8-speed transmission was commonly paired with the 5.3L V8 and the 2.7L Turbo engines.

10-Speed Automatic (10L80)

The robust 10-speed automatic was reserved for the powerful 6.2L V8 and the 3.0L Duramax Turbo-Diesel, providing optimized power delivery and efficiency.

Available Axle Ratios

The final drive ratio, or axle ratio, is critical for tailoring the truck's performance to specific needs, whether that's maximizing fuel economy or achieving the highest possible towing capacity.

How to Identify Your Axle Ratio

Knowing your truck's specific axle ratio is essential for maintenance, repairs, or when considering modifications. For GM trucks like the Sierra, the process is straightforward.

1. Check the RPO Sticker: Open the driver's side door and locate the silver "Service Parts Identification" sticker, usually found on the door jamb or inside the glove box.
2. Find the Axle Code: This sticker contains a long list of three-character Regular Production Option (RPO) codes. Scan the list for a code beginning with "G."
   • GU5 corresponds to a 3.23 ratio.
   • GU6 corresponds to a 3.42 ratio.
   • GU2 corresponds to a 3.73 ratio.

If the sticker is missing or illegible, a dealership can retrieve your truck's build sheet using the Vehicle Identification Number (VIN). Alternatively, a driveline shop can identify the ratio by inspecting the internal components of the differential.

Towing vs. MPG Impact

The axle ratio has a direct and significant impact on both towing capability and fuel efficiency. The relationship is a trade-off: what's good for one is often detrimental to the other.

• Numerically Lower Ratios (e.g., 3.23): These are often called "taller" gears. The engine spins at a lower RPM at highway speeds. This reduction in engine speed consumes less fuel, leading to better MPG. However, it provides less torque multiplication at the wheels, which reduces towing capacity and can make the truck feel less responsive when starting from a stop with a heavy load.

• Numerically Higher Ratios (e.g., 3.73): These are known as "shorter" gears. They multiply the engine's torque more effectively, making it much easier to pull heavy trailers. This is why they are included in "Max Trailering" packages and are considered best for towing. The trade-off is that the engine must run at a higher RPM at highway speeds, which increases fuel consumption and results in lower MPG.

A 3.42 ratio is often considered a "balanced" or "all-around" gear set, offering a good compromise between respectable towing performance and reasonable fuel economy for daily driving.

Common Issues

While the 2019 Sierra 1500 boasts a generally reliable driveline, some common issues can arise over the life of the vehicle, particularly under heavy use.

• Ring & Pinion Gear Whine: A whining noise from the rear differential that changes pitch with vehicle speed (not engine speed) often points to worn ring and pinion gears. This can be caused by improper gear setup, lubrication failure, or high mileage.
• Carrier Bearing Failure: A rumbling or growling noise, especially noticeable on acceleration or deceleration, can be a sign of failing carrier bearings within the differential. These bearings support the differential carrier and can wear out over time.
• Limited-Slip Clutch Wear: In models equipped with a limited-slip differential (LSD), the internal clutch packs can wear out. This typically manifests as a chattering or groaning sound from the rear end during tight, low-speed turns.

Addressing these issues promptly with a qualified technician can prevent more catastrophic and costly damage to the differential.

Find a driveline shop on Repairius.