2014 GMC Sierra 2500HD: Transmission & Axle Gear Ratios

Transmission Gear Ratios

For 2014, the GMC Sierra 2500HD was available with two heavy-duty 6-speed automatic transmissions, mated to either a gasoline or diesel engine.

6L90 (Gasoline)

Allison 1000 (Diesel)

Available Axle Ratios

The axle ratio is the final link in your truck's powertrain, multiplying the torque delivered by the engine and transmission before it reaches the wheels. Choosing the right ratio is a critical balancing act between raw towing power and long-distance fuel economy.

How to Identify Your Axle Ratio

Knowing your specific axle ratio is essential for ordering parts or understanding your truck's capabilities. There are a few ways to find this information:

• RPO Sticker: The easiest method for any GM truck is to check the "Service Parts Identification" sticker, usually located on the inside of the glove box door. This white sticker contains a list of three-character Regular Production Option (RPO) codes. Look for a code starting with "G." In the case of the 2014 Sierra 2500HD, you'll find either GT4 (3.73) or GT5 (4.10).

• VIN Decoder: Several online VIN decoders can provide a build sheet or a list of options that your truck was originally equipped with, which will include the factory-installed axle ratio.

Towing vs. MPG Impact

The relationship between axle ratios and performance is straightforward:

• Numerically Higher Ratios (e.g., 4.10): These are known as "shorter" gears. They multiply engine torque more aggressively, making it easier to get a heavy load moving from a standstill. This is ideal for maximizing your truck's tow rating and provides a more responsive, powerful feel, especially when hauling. The trade-off is that the engine must spin at a higher RPM at any given speed, which leads to increased fuel consumption.

• Numerically Lower Ratios (e.g., 3.73): These are "taller" gears. They don't multiply torque as much, requiring more effort to get started with a heavy trailer. However, once at cruising speed, the engine can operate at a lower, more efficient RPM. This results in better fuel economy, particularly on the highway. The 3.73 ratio paired with the high-torque Duramax diesel provides an excellent balance of towing capability and efficiency.

Common Issues

While the heavy-duty axles in the Sierra 2500HD are robust, they are not immune to problems, especially under heavy use or with high mileage.

• Ring & Pinion Whine: A whining noise from the rear differential that changes pitch with vehicle speed (not engine speed) often indicates worn ring and pinion gears. This can be caused by improper gear setup, lubrication failure, or simple high-mileage wear.

• Carrier Bearing Failure: A rumbling or growling noise, particularly noticeable on acceleration or deceleration, can point to failing carrier bearings. These bearings support the differential carrier within the axle housing.

• Axle Seal Leaks: Leaks from the pinion seal (where the driveshaft enters the differential) or the axle shaft seals (near the wheels) are common. If not addressed, the resulting low fluid level can lead to catastrophic internal damage.

If you suspect an issue with your truck's driveline, it's crucial to have it inspected by a qualified technician.

Find a driveline shop on Repairius.