2015 Kenworth T680 Transmission & Axle Gear Ratios

The 2015 Kenworth T680 is a staple in the heavy-duty Class 8 trucking world, renowned for its aerodynamics, fuel efficiency, and driver comfort. Whether you are running long-haul refrigerated trailers, dry vans, or heavy-haul flatbeds, the powertrain configuration—specifically the transmission and axle gear ratios—plays a crucial role in the truck's performance and profitability.

Below is a comprehensive guide to the transmission and axle gear ratios available for the 2015 Kenworth T680, along with tips on identifying your setup, understanding its impact on performance, and troubleshooting common issues.

Transmission gear ratios

The 2015 Kenworth T680 was offered with several transmission options to match the PACCAR MX-13 or Cummins ISX15 engines. The most popular choices were the Eaton Fuller Advantage 10-speed automated manual, the Eaton RTLO 13-speed, and the heavy-duty Eaton 18-speed manual.

Eaton Fuller Advantage 10-Speed Automated (FAS-10)

Eaton RTLO-18913A 13-Speed Manual

(Note: Exact internal ratios can vary slightly depending on the specific Eaton Fuller model code selected at the time of build.)

Available axle ratios

Kenworth offered a variety of tandem drive axles for the T680 in 2015, primarily from Dana Spicer, Meritor, and PACCAR's own proprietary 40K axles. The choice of rear gear heavily dictates the cruising RPM.

How to identify your axle ratio

If you are unsure of the gear ratio equipped on your 2015 Kenworth T680, you can identify it using a few simple methods:

1. Door-Jamb Data Sticker: Open the driver's side door and locate the vehicle data tag (or RPO sticker equivalent). This sticker usually lists the front and rear axle capacities, along with the specific rear axle ratio (e.g., "REAR AXLE RATIO: 3.36").
2. Axle Tag: Crawl under the rear of the truck and look at the differential housings. Both Dana and Meritor affix a metal identification tag to the differential cover or carrier housing. You might need to use a wire brush and some brake cleaner to read it. The tag will have a string of numbers indicating the model (e.g., DSP41) and the ratio (e.g., 3.25 or 3.36).
3. VIN Decoder / Dealership: Call a local Kenworth dealer with the last 6 digits of your VIN. They can pull the original build sheet, which details the exact transmission model, axle manufacturer, and gear ratios installed at the factory.

Towing vs MPG impact

In the heavy-duty trucking industry, the relationship between your rear gear ratio and your bottom line cannot be overstated.

• Numerically Lower Ratios (e.g., 2.64, 2.79): Often referred to as "tall" or "fast" gears, these are ideal for fuel economy. Combined with a direct-drive transmission, a 2.64 or 2.79 rear axle allows the engine to downspeed, cruising at 65 mph at very low RPMs (around 1,100 - 1,200 RPM). This saves a significant amount of diesel over long highway stretches but provides less low-end grunt for starting heavy loads on steep grades.
• Numerically Higher Ratios (e.g., 3.73, 4.10): Known as "short" gears, these provide massive torque multiplication at the wheels. This is better for heavy-haul, vocational, or mountainous routes. The tradeoff is reduced fuel economy, as the engine must turn at higher RPMs to maintain highway speeds.
• Balanced (3.25 - 3.55): A good middle ground for owner-operators who mix flatland highway cruising with occasional heavy loads or mountainous terrain.

Common issues

While the drivelines in the 2015 Kenworth T680 are built for severe duty, high mileage takes its toll. Watch out for these common issues:

• Ring & Pinion Whine: Often a result of improper backlash due to bearing wear over hundreds of thousands of miles, or a history of low differential fluid. A distinct whining noise on acceleration or deceleration points to ring and pinion wear.
• Carrier Bearing Wear: The multi-piece driveshafts on a T680 require steady support. A worn carrier bearing will cause a noticeable shudder or vibration through the floor of the cab, especially under load or initial acceleration.
• Power Divider / Interaxle Differential Failure: If a driver locks the interaxle differential while the wheels are actively spinning (e.g., in snow or mud), it can shear the teeth inside the power divider. Always engage the lock when stopped or rolling slowly without wheel slip.
• Input Shaft / Yoke Leaks: Pinion seal leaks are common as the truck ages. If left unchecked, the fluid loss will quickly burn up the differential bearings.

Find a driveline shop on Repairius.