Ford F-450 & F-550 Conversions

Posted by Diesel Conversion on Oct 15th 2020


The 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 3.55:13.73:1, or even 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 as 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 is prime torque at low-RPM (1600-2000 RPM) which makes the F-450/550 gearing noticeable.   Shifts will happen quickly and highway speeds may be limited.  Some customers have opted for gear and tire upgrades to keep engine RPM at a comfortable level.

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 weighs between 4,500 and 8,0000 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.