2020 Peterbilt 389: Transmission & Axle Gear Ratios

The 2020 Peterbilt 389 is an absolute icon of the American highway, combining classic styling with modern heavy-duty performance. As a Class 8 truck, it is highly customizable for anything from long-haul highway cruising to extreme heavy-haul vocational work. Because of this versatility, selecting or identifying the correct powertrain specifications—specifically the transmission and axle gear ratios—is critical for maximizing efficiency, pulling power, and drivetrain longevity.

Below is a comprehensive guide to the transmission and rear axle gear ratios available for the 2020 Peterbilt 389, how to identify what you have, and how these specifications impact your bottom line.

Transmission gear ratios

Peterbilt offers a wide array of transmission options for the 389, primarily from Eaton Fuller. The most popular choices for owner-operators and fleets running the 389 are the 13-speed and 18-speed manual transmissions, though automated manuals like the UltraShift PLUS are also available. Below are the internal gear ratios for the most common heavy-duty manuals found in this model.

Eaton Fuller RTLO-18918B (18-Speed Manual) This transmission provides extreme flexibility for heavy hauling and varied terrain.

Eaton Fuller RTLO-16913A (13-Speed Manual) A staple for highway cruising, offering great drivability and fuel economy for standard freight.

Available axle ratios

The 2020 Peterbilt 389 utilizes heavy-duty tandem rear axles, most commonly from Dana Spicer or Meritor. The optimal ratio depends heavily on the engine (Cummins X15 or PACCAR MX-13), tire size, and intended application.

How to identify your axle ratio

Unlike light-duty trucks, heavy-duty semi-trucks are built completely custom to the buyer's order, making ratio identification slightly different.

1. The Build Sticker / Door-Jamb Sticker: Open the driver’s side door and look at the Peterbilt factory build sticker. This will often list the exact front and rear axle models, weight ratings, and the gear ratio (e.g., "Ratio: 3.36").
2. Axle Tag: Climb under the rear of the truck (ensure the truck is secured and safely parked). Look for a metal plate riveted to the housing of the front-rear and rear-rear differential carriers. Meritor and Dana Spicer both stamp the exact ratio, part number, and date of manufacture on these plates.
3. VIN Decoder: Contacting a Peterbilt dealership parts department with the last 6 digits of your VIN will allow them to pull the exact factory build sheet, revealing your transmission and axle gear ratios.

Towing vs MPG impact

In the Class 8 trucking world, gear ratios dictate your engine's cruising RPM, which directly impacts your profit margins.

• Numerically lower ratios (e.g., 2.64, 3.08): These are "tall" or "fast" gears. They allow the engine to run at lower RPMs at highway speeds (usually between 1,100–1,350 RPM depending on tire size), delivering the best fuel economy. However, they provide less mechanical torque multiplication at the wheels, making them less ideal for pulling heavy grades or starting massive loads.
• Numerically higher ratios (e.g., 3.90, 4.11): These are "short" gears, designed for maximum towing capacity and low-speed pulling power. The trade-off is that your engine will be screaming at 65 MPH, severely degrading your highway MPG. These are strictly for heavy haulers, loggers, or vocational trucks that spend more time pulling hard than cruising fast.

Common issues

While Dana and Meritor tandem axles are incredibly robust, heavy payloads and high mileage can eventually take their toll:

• Power Divider (Interaxle Differential) Wear: If a driver engages the power divider while the wheels are actively spinning and losing traction, it can sheer the gears or cause catastrophic failure.
• Ring & Pinion Whine: A distinct howling or whining noise under acceleration or deceleration often indicates improper backlash, worn carrier bearings, or gear tooth wear.
• Carrier Bearing Failures: Vibrations in the driveline can often be traced back to worn steady bearing/carrier bearings on the multi-piece driveshafts connecting the transmission to the tandem axles.

Find a driveline shop on Repairius

Dealing with a howling differential, a grinding 18-speed, or looking to swap your axle ratios for better fuel economy? Heavy-duty driveline work requires specialized tools, heavy lifts, and serious expertise. Find a driveline shop on Repairius to get your 2020 Peterbilt 389 back on the road and pulling strong.