Solving Speed and Gear Ratio Challenges in a 1973 Kenworth

[MikePhua]

The 1973 Kenworth and Its Powertrain Configuration
The 1973 Kenworth tractor remains a symbol of American trucking heritage, built during an era when durability and mechanical simplicity were paramount. Often spec’d for logging, dump hauling, or mountainous terrain, these trucks were equipped with robust drivetrains tailored for torque rather than highway speed. One such example features a freshly rebuilt Cummins Big Cam engine paired with an Eaton Fuller RTO-1213 13-speed transmission and 4.33 rear axle gears, riding on 24.5-inch rubber.
Despite the engine’s capability, the truck cruises at 55 mph while turning 1,800–1,900 RPM—an indication that the gearing is optimized for pulling power, not speed. This setup, while ideal for steep grades and heavy loads, places the truck in the slow lane on modern highways.
Understanding Transmission Nomenclature and Gear Ratios
Eaton Fuller’s transmission codes reveal key details:

• RT: Roadranger Twin Countershaft
  
• RTO: Roadranger Twin Countershaft Overdrive
  
• RTLO: Roadranger Twin Countershaft Low Inertia Overdrive
  
• RTAO: Roadranger Twin Countershaft Automated Overdrive
  

The “O” in RTO confirms the presence of an overdrive gear, typically around 0.87:1. However, when paired with 4.33 rear gears, even an overdrive transmission may not deliver the desired highway speed. Using the formula:
RPM = (MPH × Axle Ratio × 336) ÷ Tire Diameter
At 55 mph with 4.33 gears and 24.5-inch tires, the RPM aligns with the observed 1,900, confirming the drivetrain’s limitations.
Exploring Transmission Swaps and Rear Gear Alternatives
To increase road speed without over-revving the engine, two primary options exist:

• Swap to a transmission with deeper overdrive, such as an RTO-14613 or RTLO-16913A, which may offer ratios as low as 0.73:1.
  
• Replace the rear axle assembly with a set of 4.10 or 3.73 gears, commonly found in late-model air ride cutoffs.
  

For example, switching to 3.73 gears with a 0.78 overdrive and 11R22.5 tires could yield:

• 55 mph at 1,600 RPM
  
• 65 mph at 1,875 RPM
  

This setup balances fuel economy and drivability, especially for long-haul applications.
Verifying Rear Axle Ratios
Axle tags may not always reflect actual gear ratios. To confirm:

• Jack up both rear wheels
  
• Mark the driveshaft and rotate the tires two full revolutions
  
• Count driveshaft turns and divide by two
  

This method accounts for spider gear movement and provides an accurate gear ratio. In one fleet example, a truck with 4.56 gears and an 18-speed ran 65 mph at 1,950 RPM—similar to the Kenworth’s current performance.
Final Drive Strategy and Use Case Considerations
Before committing to a swap, consider the truck’s intended use:

• Logging or off-road hauling benefits from low gears and high torque
  
• Highway or regional hauling demands taller gears and lower RPMs for efficiency
  
• Mixed-use operations may require a compromise, such as 4.10 gears with a moderate overdrive
  

If the truck rarely uses the lowest gear, flipping the differential or reconfiguring the transmission input can also alter final ratios, though this requires expert setup.
Conclusion
The 1973 Kenworth’s slow-lane performance is a product of its original design priorities—torque, durability, and grade-climbing ability. Modern highway demands call for taller gearing and deeper overdrive. Whether through transmission upgrades or axle swaps, the solution lies in matching drivetrain geometry to operational needs. With careful planning and accurate measurements, this classic rig can cruise confidently in today’s fast lanes without sacrificing its vintage charm.