2020 Kenworth T680 — Transmission & Axle Gear Ratios

Transmission gear ratios

The Kenworth T680 for 2020 was available with a wide array of transmission options, primarily focusing on automated manual transmissions (AMTs) for fuel efficiency and driver comfort, alongside traditional manual transmissions for specific applications. The most common AMTs include the Eaton Fuller Advantage series and the PACCAR TX-12.

Eaton Fuller Advantage 10-Speed AMT

Eaton Fuller Advantage 12-Speed AMT

PACCAR TX-12 12-Speed AMT

Available axle ratios

The 2020 Kenworth T680 offered a variety of axle ratios to cater to diverse operational needs, from long-haul fuel efficiency to heavy-haul pulling power. The availability often depended on the engine choice and specific vocational packages.

Note: RPO/Axle codes can vary significantly between Kenworth, Meritor, and Dana Spicer axles. Always verify on the vehicle's RPO sticker or axle tag.

How to identify your axle ratio

Identifying your Kenworth T680's axle ratio is crucial for proper maintenance, part replacement, and understanding your truck's performance characteristics. Here are the primary methods:

Door-jamb RPO sticker (General Motors example, less common for Kenworth for axle codes specifically)

While GM vehicles commonly use a door-jamb RPO (Regular Production Option) sticker with three-digit codes for axle ratios (e.g., "GU6" for 3.42), Kenworth trucks, like other heavy-duty manufacturers, typically do not list the axle ratio directly with an RPO code on a simple door jamb sticker. Instead, you'll find a more detailed white sticker or plate that often includes axle specifications. You will need to check your specific truck for detailed build information, which might be found on a sticker inside the driver's side door or on the firewall.

Axle Tag (Most Reliable Method)

The most reliable way to determine your axle ratio is by locating the metal identification tag attached to the axle housing itself. For Meritor and Dana Spicer axles, which are common in the Kenworth T680, these tags are typically bolted to one of the differential cover bolts or are a stamped metal tag on the axle tube. The tag will usually contain:

• Axle Model Number: Such as "Meritor MT-40-14X" or "Dana D40-170."
• Bill of Material (BOM) Number: A unique number specific to your axle configuration, which can be cross-referenced with the axle manufacturer's literature to determine the exact ratio.
• Ratio Stamping: Often, the ratio will be directly stamped on the tag in a format like "3.58" or "3 58." Sometimes, it will show the ring and pinion tooth count, for example, "41-11" (41 ring gear teeth and 11 pinion gear teeth), which calculates to a 3.73 ratio (41 ÷ 11 ≈ 3.727).

Locating this tag might require crawling under the truck. Ensure the area is clean to read the stamped information.

VIN Decoder (Less Specific for Axle Ratio)

A Vehicle Identification Number (VIN) decoder can provide general information about your truck, including the engine and transmission type. While some decoders for heavy-duty trucks might provide axle ratio range, they are often not precise enough to give the exact ratio. It's a good starting point but should be verified with the axle tag.

Towing vs MPG impact

The choice of axle ratio profoundly impacts a heavy-duty truck's performance characteristics, specifically a trade-off between towing capability and fuel economy.

• Numerically Lower Ratio (e.g., 2.47, 2.64, 2.79): Better MPG, Lower Towing Capacity. A numerically lower axle ratio means the engine turns fewer revolutions per mile for a given road speed. This reduces engine wear and consumes less fuel, making these ratios ideal for long-haul applications where the truck operates at sustained highway speeds with lighter loads. However, starting power and the ability to climb steep grades with heavy loads will be reduced, potentially requiring more frequent downshifts from the driver or the transmission.

• Numerically Higher Ratio (e.g., 3.25, 3.42, 3.58): Better Towing, Lower MPG. A numerically higher axle ratio means the engine turns more revolutions per mile. This provides more torque multiplication to the drive wheels, greatly improving acceleration from a stop, enhancing the ability to pull heavy loads, and making it easier to climb steep inclines. The downside is increased engine RPM at highway speeds, leading to higher fuel consumption and potentially more noise. These ratios are preferred for vocational applications, heavy-haul, or regional operations involving frequent starts and stops or significant grades.

Choosing the right balance depends entirely on the primary use case of the truck. Kenworth offers a range of options to optimize for specific operational profiles.

Common issues

While heavy-duty truck axles are built for durability, common issues that can arise in any differential setup include:

• Ring and Pinion Whine: Often indicative of improper gear mesh, worn gears, or insufficient lubrication. A consistent whine that changes pitch with vehicle speed but not engine RPM (in gear) is a classic symptom.
• Carrier Bearing Failure: The bearings supporting the differential carrier can wear out, leading to growling noises, especially when turning, and potentially excessive play in the driveshaft.
• Limited-Slip Clutch Wear (if equipped): For trucks equipped with limited-slip differentials, the clutch packs can wear over time, leading to a loss of limited-slip function and sometimes shuddering or chattering during turns. Most heavy-duty trucks, especially line-haul, use open differentials with a driver-controlled differential lock, making limited-slip clutch wear less common.
• Axle Seal Leaks: Over time, axle seals can degrade, leading to leaks of differential fluid. This can be identified by fluid on the inside of the wheels or around the differential housing.

Regular maintenance, including differential fluid changes at recommended intervals, is crucial for extending the life of your axles and preventing these issues.

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