2016 Peterbilt 579: Transmission & Axle Gear Ratios

Transmission Gear Ratios

The 2016 Peterbilt 579 was available with a wide range of manual and automated manual transmissions from Eaton and PACCAR. Each was designed to optimize performance and efficiency for different applications, from long-haul highway cruising to vocational jobs.

PACCAR TC-12 (Automated)

Eaton Fuller Advantage Series (Automated)

Eaton Fuller RTLO-Series (Manual, 18-Speed)

This transmission has a deep reduction gear and an auxiliary splitter, resulting in a large number of available ratios for maximum versatility.

Available Axle Ratios

Peterbilt offered a vast selection of rear axle ratios for the 579, allowing customers to specify their truck for fuel economy, pulling power, or a balance of both. Ratios were supplied by manufacturers like Dana and Meritor.

How to Identify Your Axle Ratio

Unlike passenger cars, heavy-duty trucks provide axle information in a more direct way. Here’s how to find out which ratio your Peterbilt 579 is equipped with:

1. Axle Housing Tag: The most reliable method is to locate the metal tag bolted to the differential carrier housing of the rear axle(s). This tag will have the manufacturer's name (e.g., Dana, Meritor) and will have the gear ratio stamped directly on it, often as "3.36" or "3.55". It may also include the model number and other identifiers.
2. Original Build Sheet/Spec Sheet: The sales invoice or original factory build sheet details every component of the truck, including the specific axle model and ratio that was ordered.
3. VIN Decoder (Limited): While a VIN decoder can provide general specifications, it often does not include the specific axle ratio, as this was a highly customizable option. It may provide the axle model, but the ratio itself usually needs to be confirmed on the axle tag.

Towing vs. MPG Impact

The relationship between axle ratios and performance is a fundamental concept in trucking. It’s a trade-off between pulling power (torque multiplication) and cruising speed efficiency (engine RPM).

• Numerically Lower Ratios (e.g., 2.64): These are often called "tall" or "fast" gears. The driveshaft turns fewer times for each rotation of the wheels. This allows the engine to operate at a lower RPM at highway speeds, which significantly reduces fuel consumption. These ratios are ideal for long-haul carriers operating on flat terrain with relatively light loads.

• Numerically Higher Ratios (e.g., 3.90): Known as "short" or "low" gears. The engine and driveshaft must turn many times to produce a single wheel rotation. This multiplies the engine's torque, providing exceptional power for getting heavy loads moving, climbing steep grades, and handling vocational or heavy-haul tasks. The trade-off is much higher engine RPM at highway speeds, leading to increased fuel consumption.

Choosing the right axle ratio is critical. A truck spec’d for fuel economy with a 2.64 ratio would struggle immensely if tasked with pulling a heavy load up a mountain, while a truck with a 3.90 ratio would be inefficient and burn excess fuel on a cross-country haul.

Common Issues

While the Dana and Meritor axles used in the Peterbilt 579 are robust, high-mileage and heavy-use can lead to certain issues.

• Ring & Pinion Whine: A howling or whining noise from the differential that changes pitch with vehicle speed can indicate worn ring and pinion gears. This is often caused by extended use, improper backlash, or contaminated lubricant.
• Carrier Bearing Failure: A rumbling or groaning noise, especially when turning, can point to worn carrier or pinion bearings. This can be caused by metal fatigue or lubricant breakdown over hundreds of thousands of miles.
• Limited-Slip Clutch Wear: For axles equipped with a limited-slip or locking differential, the internal clutch packs can wear out over time, reducing their effectiveness in low-traction situations.

Regularly scheduled lubricant changes with the correct spec fluid are the best way to prolong the life of any axle components.

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