2019 Peterbilt 579 Transmission & Axle Gear Ratios

Transmission gear ratios

The 2019 Peterbilt 579 was available with a variety of Eaton Fuller transmissions, including manual and automated manual (AMT) options. The specific gear ratios could vary slightly depending on the exact model and configuration, but common options are listed below. Most Peterbilt 579 trucks built in 2019 were equipped with Eaton Fuller Advantage or UltraShift PLUS transmissions.

Eaton Fuller Advantage 10-Speed Manual (FRO-15210B / FRO-16210B / FRO-17210B)

Eaton Fuller UltraShift PLUS 13-Speed AMT (FO-16E313A-VDP / FO-17E313A-VDP)

Eaton Fuller UltraShift PLUS 18-Speed AMT (RTLOF-18918B-DM3 / RTLOF-18918B-DM4)

Available axle ratios

Peterbilt offers a wide range of Meritor and Dana Spicer rear axles for the 579, allowing for customization based on application. The most common axle ratios for the 2019 Peterbilt 579 typically fall within the range of 2.47 to 3.91.

Note: "A" followed by the ratio is a common Peterbilt internal designation for axle ratios, but the actual RPO code or axle tag may have a more complex identifier.

How to identify your axle ratio

Identifying your Peterbilt 579's axle ratio is crucial for proper maintenance and performance tuning. Here are the most common methods:

• Door-jamb RPO sticker (Peterbilt-specific label): Peterbilt trucks, like other heavy-duty vehicles, often have a data sticker or plate located on the driver's side door jamb or inside the glove box. This sticker will list various RPO (Regular Production Option) codes for your specific vehicle's build. Look for codes related to the "Axle" or "Rear Axle" specification. The ratio will typically be displayed directly or encoded within a longer string. It may look like "A2.64" for a 2.64 ratio, for example.
• Axle Tag: The most definitive way to identify your axle ratio is to inspect the metal tag attached to the rear axle housing. This tag is usually bolted to one of the differential cover bolts or held by a strap. It will contain information such as the axle manufacturer (Meritor, Dana Spicer), model number, serial number, and, most importantly, the gear ratio. The ratio will be stamped directly, often in a format like "3.55" or "37:10" (which indicates 37 ring gear teeth divided by 10 pinion gear teeth, equaling 3.70).
• VIN Decoder: While not as precise for all specific axle configurations, using a commercial truck VIN decoder can often provide general specifications, including potential axle ratios based on the vehicle's original build sheet. This is a good starting point if you cannot easily access the physical tags.

Towing vs MPG impact

Choosing the correct axle ratio is a significant factor in balancing a truck's towing capability with its fuel economy. The impact can be summarized as follows:

• Numerically Lower Ratio (e.g., 2.47, 2.64, 2.79): These are often referred to as "tall" gears. They allow the engine to operate at lower RPMs at highway speeds, resulting in better fuel economy. This is ideal for long-haul applications on flat or gently rolling terrain where maximum fuel efficiency is the priority. However, a lower ratio reduces the effective torque multiplier at the wheels, making it less suitable for heavy loads or steep grades, as the engine will have to work harder and downshift more frequently.
• Numerically Higher Ratio (e.g., 3.55, 3.70, 3.91): These are often referred to as "short" gears. They increase the torque multiplication at the wheels, providing superior pulling power for heavy loads, vocational applications, or driving in mountainous terrain. The trade-off is higher engine RPMs at highway speeds, leading to decreased fuel economy. Trucks used for heavy-duty towing, construction, or demanding regional hauling will typically benefit from a higher axle ratio.
• Balanced Ratio (e.g., 3.08, 3.25): These ratios aim to strike a balance between fuel efficiency and towing capability, making them suitable for a wider range of applications, such as regional haul or general freight where both decent fuel economy and some towing power are required.

Common issues

While Peterbilt trucks and their driveline components are built for durability, certain issues can arise with axle and differential components:

• Ring & Pinion Whine: A persistent howling or whining noise that changes with road speed (not engine RPM) often indicates wear in the ring and pinion gears. This can be caused by improper setup (incorrect backlash or preload), lack of lubrication, or simply high mileage and heavy use. Ignoring this noise can lead to catastrophic failure of the differential.
• Carrier Bearing Wear: The carrier bearings support the differential carrier within the axle housing. Worn carrier bearings can manifest as a rumble, growl, or hum that might be speed-sensitive and can sometimes be felt through the floor. Severe wear can lead to excessive play, affecting gear mesh and potentially causing ring and pinion damage.
• Limited-Slip Clutch Pack Wear (if equipped): If your truck is equipped with a limited-slip differential, the internal clutch packs can wear out over time, especially with heavy use or aggressive driving. Symptoms include a chattering or clunking noise when turning tightly, or a noticeable decrease in traction when one wheel loses grip.

Find a driveline shop on Repairius.