2024 GMC Sierra 2500HD Transmission & Axle Gear Ratios

Transmission gear ratios

The 2024 GMC Sierra 2500HD offers two robust transmission options, both designed to handle the demands of heavy-duty work and towing. Each is paired with a specific engine to optimize performance.

Allison 10-speed Automatic Transmission (LGZ Duramax 6.6L Turbo-Diesel V8)

This Allison-branded transmission is a highly regarded unit, known for its smooth shifts and durability, making it ideal for the high torque output of the Duramax diesel engine.

GM 10-speed Automatic Transmission (L8T 6.6L V8 Gas)

Paired with the gasoline V8 engine, this 10-speed automatic transmission provides a wide range of gears to ensure optimal power delivery and efficiency across various driving conditions.

Available axle ratios

The 2024 GMC Sierra 2500HD offers a limited but effective selection of axle ratios, primarily depending on the chosen engine and intended use. The axle ratio plays a crucial role in balancing towing capability with fuel economy.

How to identify your axle ratio

Knowing your truck's axle ratio is important for various reasons, from proper maintenance to understanding its performance characteristics. For the 2024 GMC Sierra 2500HD, the easiest way to identify your axle ratio is through the RPO (Regular Production Option) sticker.

GM RPO Sticker: Locate the RPO sticker, typically found on the inside of the glove box door or sometimes on the driver's side door jamb. This sticker contains a list of three-character codes that represent all the options your vehicle was built with. Look for codes starting with "GT".

• GT4: Indicates a 3.73 rear axle ratio.
• GT5: Indicates a 3.42 rear axle ratio.

If the RPO sticker is missing or unreadable, you might be able to find an axle tag attached to one of the differential cover bolts on the rear axle housing. This metal tag will usually have the gear ratio stamped on it. As a last resort, a GMC dealership parts department can often provide this information using your VIN.

Towing vs MPG impact

The axle ratio has a direct and significant impact on both your towing capability and fuel economy. It's a fundamental trade-off in vehicle design:

• Numerically Higher Axle Ratio (e.g., 3.73 compared to 3.42): This means the driveshaft will spin more times for each rotation of the wheels. This provides more torque multiplication at the wheels, which is excellent for towing heavy loads, launching, and accelerating. However, it also means the engine will be revving at higher RPMs at any given road speed, leading to lower fuel economy, especially at highway speeds.

• Numerically Lower Axle Ratio (e.g., 3.42 compared to 3.73): In contrast, a lower ratio means the driveshaft spins fewer times per wheel rotation. This results in less torque multiplication but allows the engine to operate at lower RPMs at highway speeds, leading to better fuel economy. The trade-off is reduced towing capacity and slower acceleration, especially with heavy loads.

For the 2024 GMC Sierra 2500HD, the 3.73 ratio is standard with the gasoline engine to provide ample towing power, while the Duramax diesel, with its inherently higher torque output, can achieve excellent towing performance with the more fuel-efficient 3.42 ratio.

Common issues

While GMC Sierra HD axles and transmissions are generally robust, like any mechanical components, they can experience issues over time. Being aware of these can help with early diagnosis.

• Rear Axle Whine: A common complaint can be a whining or howling noise from the rear axle, particularly when decelerating or at certain speeds. This often indicates wear in the ring and pinion gears or improper backlash adjustment. If left unaddressed, it can lead to more severe damage.

• Carrier Bearing Noise: The carrier bearings support the differential carrier within the axle housing. Worn or failing carrier bearings can produce a growling or rumbling sound that often changes with vehicle speed and can be heard from the rear of the truck. This requires prompt attention to prevent differential failure.

• Limited-Slip Differential (LSD) Wear: If your Sierra 2500HD is equipped with a limited-slip differential, the clutch packs within the LSD can wear out over time. Symptoms might include chattering or clunking sounds from the rear end, especially when turning at low speeds, or a noticeable reduction in traction in slippery conditions. Regular fluid changes with the correct friction modifier are crucial for LSD longevity.

• Transmission Shifting Issues: While the Allison and GM 10-speed transmissions are highly rated, any automatic transmission can develop issues. Symptoms might include harsh shifts, delayed engagement, slipping, or a reluctance to shift into certain gears. These can be caused by low or contaminated fluid, solenoid issues, or internal wear. Prompt diagnosis by a qualified technician is recommended.

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