Can You Run Float Tires on a 1974 Dodge CM900 Without Major Modifications

[MikePhua]

The Dodge CM900 and Its Role in Heavy Haul History
The 1974 Dodge CM900 was part of Chrysler’s commercial truck lineup during an era when Dodge was still producing medium- and heavy-duty trucks for vocational use. Built with Rockwell axles and robust steel frames, the CM900 was designed for logging, oilfield, and municipal work. Though Dodge exited the heavy truck market in the late 1970s, the CM-series trucks remain in use among collectors and rural operators who value their simplicity and ruggedness.
With a front axle rating often around 12,000 lbs and a cab-over design, the CM900 offered good visibility and maneuverability for its time. However, adapting it to modern tire configurations—especially wide float tires—requires careful consideration of axle geometry, wheel offset, and bearing load limits.
What Are Float Tires and Why Use Them
Float tires, or flotation tires, are wide-profile tires designed to distribute weight over a larger surface area. They are commonly used in agriculture, oilfield transport, and soft terrain hauling to reduce ground pressure and improve traction.
Typical float tire specs:

• Width: 385 mm to 445 mm (15"–17.5")
  
• Rim diameter: 22.5" or 24.5"
  
• Load rating: 6,000–8,000 lbs per tire
  
• Recommended wheel offset: 2"–5" outward depending on axle clearance
  

Benefits include:

• Reduced rutting and soil compaction
  
• Improved ride quality on rough terrain
  
• Enhanced stability when hauling heavy loads
  

However, installing float tires on older trucks like the CM900 isn’t as simple as swapping rims.
Wheel Offset and Bearing Load Concerns
The key challenge in running float tires on a CM900 is wheel offset. Offset refers to the distance between the wheel’s mounting surface and its centerline. Increasing offset to accommodate wider tires pushes the load outward, increasing leverage on the wheel bearings.
Risks of excessive offset:

• Accelerated bearing wear
  
• Increased steering effort
  
• Reduced turning radius
  
• Potential for spindle or hub damage
  

The CM900’s front axle—likely a Rockwell 12,000-lb unit—was not designed for extreme offsets. While it can tolerate modest changes, pushing beyond factory specs may lead to premature failure.
Solutions and Recommendations
To safely run float tires on a CM900:

• Consult a tire specialist familiar with heavy truck offsets
  
• Use wheels with moderate offset (2"–3") and reinforced hubs
  
• Upgrade wheel bearings to higher load-rated equivalents if available
  
• Consider installing hub spacers with integrated bearing support
  
• Verify clearance between tire and suspension components at full lock
  

In Minnesota, a hauler retrofitted his 1976 CM-series truck with 425/65R22.5 floats using custom steel wheels with a 2.5" offset. He also upgraded to Timken tapered bearings and added gussets to the spindle arms. The result was a stable ride with no bearing failures after 40,000 miles of mixed terrain hauling.
Historical Anecdote and Modern Adaptation
In the 1980s, float tires became popular among oilfield haulers in Alberta who needed to traverse muddy lease roads. Many retrofitted older trucks like the Ford L9000 and Dodge CM-series with floats, often welding custom brackets and modifying fenders. These field adaptations laid the groundwork for today’s wide-base tire standards.
Today, float tires are common on tri-axle dump trucks, lowboys, and agricultural rigs. Manufacturers like Michelin, Goodyear, and Continental offer purpose-built float tires with reinforced sidewalls and optimized tread for off-road use.
Final Thoughts
Running float tires on a 1974 Dodge CM900 is possible—but not without mechanical and safety considerations. The key lies in understanding wheel offset, bearing load limits, and axle geometry. With proper planning and expert input, operators can enjoy the benefits of flotation without compromising the integrity of their vintage truck. As with any modification, the goal is to enhance capability while respecting the engineering limits of the original design.