2018 Kenworth T680 Transmission & Axle Gear Ratios: Complete Guide

The 2018 Kenworth T680 is a flagship aerodynamic heavy-duty truck designed primarily for long-haul highway efficiency. Powertrain optimization is critical in the Class 8 world, where minor adjustments to gearing can result in thousands of dollars in fuel savings or drastically alter heavy-hauling capabilities. By perfectly matching the engine’s power band to the transmission internal ratios and the rear axle ratio, fleets and owner-operators can maximize their profitability.

Below is a comprehensive guide to the transmission options and axle gear ratios available for the 2018 Kenworth T680.

Transmission gear ratios

The 2018 Kenworth T680 was offered with several transmission choices, primarily dominated by the Eaton Fuller and PACCAR families. The push for automated manual transmissions (AMTs) was in full swing by 2018, though traditional manuals remained a staple for specific applications.

PACCAR TX-12 (12-Speed Automated)

Designed to pair seamlessly with the PACCAR MX-13 and MX-11 engines, this lightweight AMT is optimized for linehaul fuel efficiency.

Eaton Fuller Advantage 10-Speed Automated (FAS)

A highly popular choice for regional and long-haul fleets seeking reliability and optimized shifting logic.

Eaton Fuller 13-Speed Manual (RTLO-16913A)

For owner-operators and heavy-haul applications needing precise driver control over split gears.

Available axle ratios

The T680 typically utilizes tandem rear axles from Dana Spicer or Meritor. The chosen ratio dictates the truck’s cruising RPM. In 2018, the industry trended heavily toward "downspeeding"—using numerically lower ratios to keep engine RPMs low at highway speeds.

How to identify your axle ratio

Identifying the exact axle ratio on your 2018 Kenworth T680 is essential before modifying tires, reprogramming the ECM, or ordering replacement driveline components.

• Door-Jamb Data Plate: Look inside the driver’s side door jamb. Kenworth affixes a primary build decal here that lists the gross axle weight ratings (GAWR) along with the installed transmission model and rear axle ratio.
• Axle Tag: Crawl beneath the rear of the truck (following all safety protocols) and inspect the differential carriers. Both Dana Spicer and Meritor attach a metal identification tag directly to the axle housing via two bolts or heavy-duty adhesive. This tag will list the model number (e.g., DSP41) and the exact ratio (e.g., 2.64).
• VIN Decoder / Dealership: You can provide your 17-character Vehicle Identification Number (VIN) to any authorized Kenworth dealership. Their internal system will pull the exact factory build sheet, verifying the original gear sets.

Towing vs MPG impact

The relationship between your axle ratio and your truck's performance is a balancing act between pulling power and fuel conservation.

Numerically Lower Ratios (e.g., 2.64, 2.79): These are known as "fast" or "tall" gears. They are best for fuel economy. By utilizing engine downspeeding, a 2.64 ratio allows a PACCAR MX-13 engine to cruise at 65 MPH while turning at a highly efficient 1,100 to 1,200 RPM. However, these ratios offer less mechanical leverage, making them poorly suited for heavy-haul logging or continuous steep mountain climbing.

Numerically Higher Ratios (e.g., 3.55, 3.70): These are "slow" or "short" gears, prioritizing towing and heavy load capability. They provide excellent startability and gradeability because the engine turns more times per wheel revolution, multiplying torque. The trade-off is reduced fuel economy, as the engine will spin at much higher RPMs at highway cruising speeds.

Common issues

Class 8 axles and transmissions are built for extreme durability, often lasting over a million miles with proper maintenance, but they are not entirely immune to wear and tear.

• Ring & Pinion Whine: A high-pitched whining noise under acceleration or deceleration usually indicates incorrect backlash or worn ring and pinion gears. This often stems from prolonged heavy hauling or inadequate lubrication intervals.
• Carrier Bearing Wear: Vibrations felt through the chassis or a low-pitch rumble can point to worn carrier bearings. If ignored, this can lead to catastrophic differential failure.
• Power Divider (Inter-Axle Differential) Failure: On tandem axles, the power divider allows differences in speed between the front and rear drive axles. Engaging the power divider lock while the wheels are actively slipping (spinning) is a common driver error that instantly destroys the power divider gears.
• Limited-Slip Clutch Wear: If your T680 is equipped with limited-slip or locking differentials, the internal clutch packs or locking mechanisms can wear over time, leading to chattering around corners or a failure to lock when traction is lost.

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