2020 Peterbilt 579: Transmission & Axle Gear Ratios

Transmission Gear Ratios

The 2020 Peterbilt 579 was offered with a variety of automated and manual transmissions, primarily from PACCAR and Eaton. Each transmission has a unique set of internal gear ratios designed to optimize the performance of the PACCAR MX-13 and MX-11 engines.

PACCAR TX-12 (Automated)

This is a 12-speed automated manual transmission, standard on many 579 models. It is designed for optimal fuel economy and smooth shifting.

Eaton Endurant (Automated)

The 12-speed Eaton Endurant is another popular automated option known for its lightweight design and reliability.

Eaton Fuller (Manual)

Manual transmissions from Eaton Fuller were available in 10, 13, and 18-speed configurations, offering drivers maximum control. The ratios below are for the common RTLO-18918B 18-speed.

Available Axle Ratios

The rear axle ratio is a critical component that multiplies the torque coming from the transmission before it reaches the wheels. Peterbilt offers a wide range of Dana and Meritor axle ratios to suit different applications.

How to Identify Your Axle Ratio

Identifying your truck's specific axle ratio is crucial for maintenance, repairs, or performance modifications. There are a few ways to find this information:

• VIN Decoder: The most reliable method is to use a commercial VIN decoding service that specializes in heavy trucks. This will provide a full build sheet for your specific 2020 Peterbilt 579, including the exact axle model and ratio it left the factory with.
• Axle Tag: Most drive axles are fitted with a metal identification tag, usually bolted to one of the differential cover bolts. This tag contains stamped information including the manufacturer (e.g., Dana, Meritor), the model number, and the gear ratio (e.g., "3.55"). It may be covered in road grime and require cleaning to read.
• OEM Build Sheet: If you are the original owner or have access to the original sale documents, the full specifications sheet will list the axle ratio.

Unlike passenger vehicles, commercial trucks like the Peterbilt 579 do not typically list axle ratio codes on a door-jamb sticker.

Towing vs. MPG Impact

The axle ratio has a direct and significant impact on both fuel economy and towing capability. The relationship is a trade-off:

• Numerically Lower Ratios (e.g., 2.26, 2.64): These are often called "tall" gears. They result in lower engine RPM at highway speeds. This reduction in engine speed decreases fuel consumption, making these ratios ideal for long-haul, on-highway applications where the truck spends most of its time in top gear. The trade-off is reduced torque multiplication, meaning less power for starting heavy loads or climbing steep grades.

• Numerically Higher Ratios (e.g., 3.70, 4.10): These are known as "short" gears. They multiply the engine's torque more effectively, providing superior pulling power for getting heavy loads moving and maintaining speed on inclines. This makes them ideal for heavy-haul or vocational applications. The downside is that the engine must turn at a higher RPM to achieve the same highway speed, leading to increased fuel consumption.

Choosing a "balanced" ratio like a 3.08 or 3.25 provides a good compromise for fleets that run a mix of highway and city routes with varying loads.

Common Issues

While the drive axles used in the Peterbilt 579 are generally robust, they are subject to immense stress. Some issues that can arise over many miles of service include:

• Ring & Pinion Whine: A whining or howling noise that changes pitch with vehicle speed can indicate a problem with the gear mesh between the ring and pinion gears. This can be caused by improper backlash, worn bearings, or contaminated lubricant.
• Carrier Bearing Failure: The carrier bearings support the differential case within the axle housing. A failing carrier bearing often produces a rumbling or growling noise that may be more pronounced when turning.
• Limited-Slip Clutch Wear: For axles equipped with a limited-slip differential, the internal clutches can wear out over time. This will result in reduced traction in slippery conditions, where one wheel may spin freely while the other receives little power.

Regularly changing the differential fluid with the correct type and weight of gear oil is the single most important step in maximizing axle life.

Find a driveline shop on Repairius.