Ford® F-450 & F-550 Conversions
Posted by Diesel Conversion Specialists® on Oct 15th 2020
The Ford® Super Duty® F-450 & F-550 trucks are a popular choice for conversion projects. These vehicles combine the amenities of a modern pickup truck with very impressive towing and hauling capabilities. The following information highlights some of the pertinent differences in the F-450/550 trucks as compared to Ford’s standard F-250/350 models.
Gear Ratios and Tire size: The larger Super Duty trucks offered axle ratios of 4.30:1, 4.88:1, and even 5.38:1. Because a higher ratio number also means higher engine RPM, these ratios are considered deep as compared to an F-250’s 3.55:1, 3.73:1, or 4.10:1 ratios.
Stock tire size for an F-450/550 is typically 225/70/R19.5, which equates to just under a 32” tall tire. This is a smaller diameter tire compared to 33”-35” and taller tires commonly found on today’s pickup trucks. The shorter tire will contribute to higher engine RPM particularly when paired with steep gearing.
Highway RPM Comparison:
Typical Ford pickup — 3.73 ratio, 32” tires, 65 MPH @ 1757 RPM (in Overdrive)
Ford F-450 — 4.88 ratio, 32” tires, 65 MPH @ 2365 RPM (in Overdrive)
The Cummins® engine makes its prime torque at low RPM (1600–2000 RPM) which makes the F-450/550 gearing something to consider. Shifts will happen quickly and highway speeds may be limited with the stock gearing and tires. However, you can move a lot of weight as was intended for the F-450/550. Some customers have opted for gear and tire changes to keep engine RPMs at a level that better fits their needs for the truck’s performance.
Overall Truck Weight and Chassis Components: The F-450/550 load ratings are due to their larger axles, brakes, springs, and chassis components. This results in a vehicle that can easily weigh 10,000–14,000 pounds. The F-250 & F-350 trucks in basic form primarily weigh between 4,500 and 8,000 pounds.
Service boxes, tow beds, and commercial equipment mounted on a truck will add even more weight.
The F-450/550 trucks will accelerate, brake, and generally handle differently than a conventional pickup truck.