2015 Peterbilt 579 — Transmission & Axle Gear Ratios

The 2015 Peterbilt 579 is a flagship aerodynamic Class 8 truck known for its fuel efficiency, driver comfort, and versatility. Whether you are running long-haul interstate routes or pulling heavy loads regionally, the powertrain configuration—specifically the transmission and axle gear ratios—dictates how well the truck performs. Understanding these ratios is crucial for maximizing fuel economy or optimizing pulling power.

Below is a comprehensive guide to the transmission and axle gear ratios available for the 2015 Peterbilt 579.

Transmission gear ratios

The 2015 Peterbilt 579 was commonly spec’d with Eaton Fuller manual transmissions or Eaton UltraShift Plus automated manual transmissions (AMTs). Below are the internal gear ratios for two of the most popular configurations: the Eaton Fuller 10-speed (FRO-16210C) and the Eaton Fuller 13-speed (RTLO-16913A), both standard choices paired with PACCAR MX-13 or Cummins ISX15 engines.

Eaton Fuller 10-Speed Manual (FRO-16210C)

Eaton Fuller 13-Speed Manual/UltraShift (RTLO-16913A)

Available axle ratios

The 2015 Peterbilt 579 utilizes industry-leading drive axles primarily from Dana Spicer and Meritor. Axle ratios typically range from ultra-fast "downsped" ratios for highway cruising to lower (numerically higher) ratios for heavy vocational or mountainous hauling.

How to identify your axle ratio

If you have purchased a used 2015 Peterbilt 579, verifying the exact axle ratio is essential before modifying the powertrain or replacing driveline components. Because heavy-duty trucks are highly customized, there are a few surefire ways to check your gear ratio:

• Door-Jamb Data Sticker / Line Sheet: Check the driver’s side door jamb. Unlike passenger trucks that use standardized RPO codes, Peterbilt issues a heavy-duty line sheet (often kept in the glove box or stuck to the door jamb) that lists the specific Dana or Meritor axle model and the exact ratio it was built with.
• Axle Tag: Crawl under the rear of the truck and look at the differential housing. Meritor and Dana Spicer stamp a metal identification tag bolted to the carrier housing. This tag will display the axle model (e.g., DSP40), the serial number, and the gear ratio (e.g., 3.36).
• VIN Decoder / Dealership: You can contact any Peterbilt dealership with your full 17-digit VIN. They can pull the original build sheet, which will detail the exact transmission model, axle manufacturer, and rear gear ratio.

Towing vs MPG impact

When evaluating heavy-duty truck gear ratios, the basic rule of thumb is:

• Numerically lower ratio (e.g., 2.64, 3.08): These "fast" gears are better for fuel economy (MPG). They are popular in downsped drivetrains, allowing the engine to cruise at interstate speeds (65–70 mph) at very low RPMs (often around 1,100 RPM). While great for flatbed or dry van highway running, they are slower to accelerate when pulling heavy loads.
• Numerically higher ratio (e.g., 3.90, 4.11): These "slow" or "deep" gears are better for heavy towing. They provide massive wheel torque, making it easier to pull heavy loads up steep grades or get massive weight moving from a dead stop. The trade-off is higher cruising RPMs on the highway, which drastically reduces fuel efficiency.
• Balanced (e.g., 3.36, 3.55): A middle-ground ratio gives regional haulers the ability to pull varying loads over rolling terrain without completely sacrificing fuel economy.

Common issues

Heavy-duty axles like the Meritor MT-40 and Dana Spicer DSH40 are robust, but a truck logging hundreds of thousands of miles will eventually experience wear. Common issues to look out for on a 2015 Peterbilt 579 include:

• Ring and Pinion Whine: As the gears wear or if the fluid runs low, you may hear a high-pitched whine from the rear axles under acceleration or deceleration. This typically indicates improper backlash or worn gear teeth.
• Carrier Bearing Failure: Rumbling or vibration at highway speeds can be traced to worn carrier bearings in the differential or worn center support bearings on the driveline.
• Power Divider / Interaxle Differential Issues: Improperly engaging the interaxle lock while the wheels are slipping can snap the power divider gears, a very common issue during winter operations.
• Wheel Seal Leaks: Leaking axle seals can drain the differential housing of gear oil, leading to catastrophic overheating and bearing failure if left unaddressed.

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