2018 Chevrolet Silverado 2500HD Transmission & Axle Gear Ratios Guide

The 2018 Chevrolet Silverado 2500HD is a heavy-duty workhorse renowned for its impressive towing capacity, payload capabilities, and robust powertrain options. Whether you are hauling heavy equipment to a job site or towing a fifth-wheel camper cross-country, understanding the gearing of your truck is critical. The gear ratios inside the transmission and the differential dictate how engine torque is multiplied and delivered to the wheels, directly impacting acceleration, towing capacity, and fuel economy.

Below is a comprehensive guide to the transmission and axle gear ratios for the 2018 Chevy Silverado 2500HD.

Transmission gear ratios

For the 2018 model year, the Silverado 2500HD was offered with two distinct powertrain combinations, each paired with a heavy-duty 6-speed automatic transmission. The 6.0L Vortec V8 gas engine is mated to GM's 6L90 transmission, while the legendary 6.6L Duramax Turbo-Diesel V8 is paired with the renowned Allison 1000 transmission.

GM 6L90 6-Speed Automatic (Standard with 6.0L V8 Gas)

The 6L90 features a deep first gear for getting heavy loads moving off the line, combined with two overdrive gears for highway cruising.

Allison 1000 6-Speed Automatic (Standard with 6.6L Duramax Diesel)

The Allison 1000 is built to handle the massive 910 lb-ft of torque generated by the L5P Duramax engine. Its ratios are spaced slightly closer to keep the diesel engine in its narrow, optimal torque band.

Available axle ratios

Unlike half-ton trucks that offer a wide array of axle ratios for various fuel economy and towing packages, heavy-duty trucks like the 2500HD keep things simple and purpose-built. The rear axle ratio on the 2018 Silverado 2500HD is strictly dictated by the engine choice.

Note: Almost all 2018 Silverado 2500HD models are also equipped with the Eaton heavy-duty automatic locking rear differential, identifiable by the G80 RPO code.

How to identify your axle ratio

If you are unsure of your exact powertrain configuration, identifying your axle ratio on a General Motors vehicle is highly straightforward thanks to the Service Parts Identification (SPID) sticker.

• Door-Jamb or Glove Box RPO Sticker: For 2018 GM trucks, you will typically find a silver Service Parts Identification sticker located inside the glove box (or sometimes on the driver’s side door jamb). This sticker contains a list of 3-character alphanumeric codes. Look for GT4 (3.73) or GT5 (4.10). You should also look for G80, which indicates you have the locking rear differential.
• Axle Tag: If the sticker is missing or unreadable, you can crawl underneath the rear of the truck. GM heavy-duty axles (usually a 10.5-inch or 11.5-inch AAM rear end) will have a metal tag bolted to the differential cover or a sticker on the axle tube that lists the ratio (e.g., "3.73" or "4.10").
• VIN Decoder: You can plug your 17-digit Vehicle Identification Number (VIN) into a reputable online VIN decoder or call a local Chevrolet dealership's parts department. They can pull up the original build sheet for your exact truck.

Towing vs MPG impact

The relationship between your axle gear ratio, towing capability, and fuel economy is a matter of physics and leverage.

• Higher Numerically (e.g., 4.10): A 4.10 ratio means the driveshaft turns 4.10 times for every one complete rotation of the tires. This provides more torque multiplication at the wheels, making it much easier to pull heavy trailers off the line and up steep grades. The trade-off is that the engine will spin at higher RPMs at highway speeds, which generally reduces fuel economy. This is why it is standard on the gas engine, which makes its power higher in the RPM band.
• Lower Numerically (e.g., 3.73): A 3.73 ratio provides slightly less mechanical leverage but allows the engine to run at lower RPMs during highway cruising, potentially improving MPG. The 6.6L Duramax diesel engine produces immense low-end torque, meaning it does not need the aggressive 4.10 gearing to get massive loads moving. The 3.73 ratio pairs perfectly with the diesel to offer exceptional towing while maintaining decent highway fuel economy.

Common issues

The driveline components in the 2018 Silverado 2500HD are generally very robust, especially the massive American Axle Manufacturing (AAM) 11.5-inch rear ends used behind the Duramax. However, heavy towing and high mileage can eventually take their toll:

• Ring & Pinion Whine: If you hear a high-pitched whining noise from the rear of the truck under acceleration or deceleration, it often indicates that the ring and pinion gears have worn or that the backlash has fallen out of specification.
• Carrier Bearing Wear: On models with a two-piece driveshaft (common on Crew Cab / Long Box configurations), the center support carrier bearing can fail, causing a noticeable shudder or vibration on takeoff.
• G80 Locker Issues: The Eaton "Gov-Lock" automatic locking differential (RPO G80) uses a system of flyweights and clutches to lock the axles when wheel slip is detected. With extreme abuse or lack of fluid maintenance, the limited-slip clutch packs can wear out, or the locking mechanism can shear, leading to a loud clunking noise or a failure to lock the rear wheels in slippery conditions.

If you suspect your heavy-duty truck is suffering from differential whine, transmission slippage, or driveline vibrations, it is best to have it inspected by a professional.

Find a driveline shop on Repairius.