2021 Kenworth T680: Transmission & Axle Gear Ratios

Transmission Gear Ratios

The 2021 Kenworth T680 was available with a wide array of transmissions from top manufacturers like Eaton and PACCAR. Each transmission offers a different set of internal gear ratios tailored to specific applications, from long-haul efficiency to vocational hauling power.

PACCAR TX-12 (12-Speed Automated)

This is Kenworth's proprietary automated manual transmission, designed for optimal efficiency when paired with the PACCAR MX engines.

Eaton Endurant (12-Speed Automated)

The Endurant is a popular automated manual transmission known for its smooth shifting and fuel economy performance.

Eaton Fuller RTLO-Series (18-Speed Manual)

A legendary choice for heavy-haul and vocational applications, the 18-speed offers maximum control and gear selection for any situation.

Note: The 18-speed features high/low range and splitter buttons, resulting in 18 forward gears. The ratios shown are for the main gears.

Available Axle Ratios

The axle ratio is the final piece of the driveline puzzle, determining how many times the driveshaft turns for every one rotation of the wheels. Selecting the right axle ratio is critical for balancing fuel economy and pulling power.

How to Identify Your Axle Ratio

Knowing your truck's specific axle ratio is crucial for maintenance, repairs, and performance analysis. Here’s how to find it:

• Axle Tag: Most heavy-duty axles from manufacturers like Meritor and Dana have a metal tag bolted to the differential housing. This tag will contain information about the model, serial number, and, most importantly, the gear ratio (e.g., "3.36").
• VIN Decoder: While not always 100% accurate for all details, a comprehensive VIN decoder service can often provide the factory-installed axle ratio and other driveline specifications.
• Dealership or OEM Build Sheet: Providing your VIN to a Kenworth dealership can allow them to pull the original build sheet, which lists every component installed at the factory, including the exact axle model and ratio.

Unlike passenger vehicles, commercial trucks do not typically list axle ratio information on a door-jamb sticker.

Towing vs. MPG Impact

The relationship between axle ratios, towing capacity, and fuel economy is a game of trade-offs.

• Numerically Lower Ratios (e.g., 2.64, 2.79): These are known as "tall" gears. The engine turns at a lower RPM at highway speeds, which reduces fuel consumption. This makes them ideal for long-haul carriers who spend most of their time on flat ground at a constant speed. The trade-off is reduced starting torque and less pulling power on steep grades.

• Numerically Higher Ratios (e.g., 3.73, 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 climbing hills. This is essential for heavy-haul, vocational, and regional applications. The downside is that the engine runs at a higher RPM on the highway, leading to increased fuel consumption.

For the T680, a driver focused on maximizing miles per gallon would choose a PACCAR TX-12 transmission paired with an MX-13 engine and a 2.64 axle ratio. Conversely, a driver hauling heavy equipment in a mountainous region would benefit from an 18-speed manual and a 3.91 or higher axle ratio.

Common Issues

The driveline components in the Kenworth T680 are built for durability, but they are not immune to wear and tear under heavy use. Some issues that can arise include:

• Pinion and Carrier Bearing Wear: A constant whining or howling noise from the rear axles that changes with vehicle speed is a classic symptom of worn pinion or carrier bearings. This is often caused by high mileage, heavy loads, or contaminated gear oil.
• Ring & Pinion Gear Whine: If the whine occurs only during acceleration or deceleration, it may point to an improper setup or wear in the ring and pinion gears themselves. This requires specialized tools to correct the gear mesh pattern.

Regularly changing the differential fluid with the manufacturer-specified type is the most critical step in maximizing the life of your axle components.

Need to service your T680's driveline? Find a driveline shop on Repairius.