2019 Kenworth T680 — Transmission & Axle Gear Ratios

Transmission gear ratios

The 2019 Kenworth T680, a popular choice for long-haul and vocational applications, was available with a variety of transmission options to suit diverse operational needs. The most common transmissions included Eaton Fuller Advantage series automated manual transmissions (AMT), Eaton Fuller manual transmissions, and Allison automatic transmissions for specific vocational applications. Below are typical gear ratios for some of the common transmissions found in the 2019 T680.

Eaton Fuller Advantage 10-Speed AMT (e.g., FO-10C310B-VCS)

Eaton Fuller Advantage 12-Speed AMT (e.g., FO-12C410B-VCS)

Eaton Fuller Advantage 18-Speed Manual (e.g., RTLO-18918B)

Available axle ratios

The Kenworth T680 could be equipped with a range of axle ratios depending on the intended application, engine choice, and customer specifications. These ratios significantly impact the truck's performance, affecting everything from fuel economy to towing capacity.

How to identify your axle ratio

Identifying the correct axle ratio for your 2019 Kenworth T680 is crucial for proper maintenance and parts replacement. There are several ways to determine this information:

• Door-Jamb RPO Sticker (GM style): While Kenworth doesn't directly use the "RPO" (Regular Production Option) sticker in the same way GM does for passenger vehicles, commercial trucks like the T680 will have a Vehicle Information Label or Certification Label typically located on the driver's side door jamb or inside the driver's side door. This label contains crucial information, including VIN, gross vehicle weight ratings (GVWR), and often, codes related to axles and other powertrain components. You may need to cross-reference these codes with a Kenworth parts catalog or a dealership to decipher the exact axle ratio.
• Axle Tag: The most reliable method is to locate the metal tag attached to the axle housing itself. This tag, usually bolted to one of the differential cover bolts or secured to the axle tube, will have the axle manufacturer's information (e.g., Meritor, Dana), the model number of the axle, and often the gear ratio stamped or etched directly on it. For tandem axles, check both the forward and rear drive axles. Be prepared that these tags can be dirty or corroded, requiring cleaning to read.
• VIN Decoder: Many truck dealerships and independent heavy-duty truck parts suppliers have access to VIN decoders that can provide a detailed breakdown of your truck's original build specifications, including axle ratios. Provide your full 17-digit VIN.
• Dealership Service Department: A Kenworth dealership service department can look up your truck's specifications using your VIN and provide you with the factory-installed axle ratios.

Towing vs MPG impact

The rear axle ratio plays a pivotal role in balancing a heavy-duty truck's fuel economy and its ability to tow or haul heavy loads. This is a fundamental trade-off in driveline specification:

• Numerically Lower Ratio (e.g., 2.47, 2.64): Better MPG, Less Towing Power. A numerically lower axle ratio means that the driveshaft makes fewer revolutions for each revolution of the wheel. This allows the engine to operate at lower RPMs at highway speeds, significantly improving fuel efficiency. This setup is ideal for long-haul applications where the truck operates predominantly on flat terrain with lighter loads, prioritizing fuel economy above all else. However, with heavy loads or on steep grades, the engine may struggle to maintain speed, requiring more downshifting and potentially leading to higher fuel consumption in those specific conditions.
• Numerically Higher Ratio (e.g., 3.42, 3.58): Better Towing Power, Lower MPG. Conversely, a numerically higher axle ratio means the driveshaft makes more revolutions for each revolution of the wheel. This provides greater torque multiplication at the wheels, enhancing the truck's ability to start from a standstill with heavy loads, climb grades, and generally handle severe-duty applications. The engine will operate at higher RPMs at any given road speed, leading to increased fuel consumption. This setup is preferred for vocational trucks, heavy haulers, and applications frequently encountering challenging terrain or very heavy gross vehicle weights.

Choosing the right axle ratio involves carefully considering the primary application of the truck. A T680 used for regional distribution with varying loads might opt for a balanced ratio like 2.79 or 3.08, while a dedicated long-haul truck with light freight would benefit from the economy of a 2.47 or 2.64. Heavy vocational use often dictates 3.25 or higher.

Common issues

While Kenworth T680 driveline components are generally robust, like any heavy-duty system, they can experience wear and issues over time. Common problems related to axles and differentials include:

• Ring & Pinion Whine: A persistent howling or whining noise that changes with road speed, especially under load or on deceleration, often indicates wear or improper setup of the ring and pinion gears within the differential. This can be caused by inadequate lubrication, improper backlash adjustment, or simply high mileage wear.
• Carrier Bearing Failure: The carrier bearings support the differential carrier, which houses the ring gear and spider gears. Failure of these bearings can manifest as a rumble, growl, or clunking sound, especially when starting or changing direction. Ignoring this can lead to catastrophic differential failure.
• Limited-Slip or Locker Clutch/Component Wear: If your T680 is equipped with a limited-slip differential or a driver-controlled differential lock (often found in vocational specs), the clutches or engagement mechanisms can wear over time. This can result in a loss of traction in low-traction conditions, grinding noises during turns (for limited-slip), or failure for the differential lock to engage or disengage properly.
• Axle Seal Leaks: Leaks from the pinion seal or axle shaft seals are common and, if left unaddressed, can lead to insufficient differential fluid, causing premature wear of internal components.

Regular maintenance, including checking differential fluid levels and quality, is essential for extending the life of your axle components.

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