Load capacity first
Federal rule 49 CFR 571.110 sets up the tire placard, and the USTMA guide says never exceed the tire's load or pressure limits. On a heavy truck, a big off-road tire with too low a load rating is a real risk. NHTSA's interpretation letter explains how placards apply after changes, so keep the original information and ask the tire maker for pressures.
Clearance
- Check at full lock and at full suspension travel, not just parked.
- Wheel offset and backspacing move the tire in or out; they change scrub radius and clearance.
- Bigger, heavier tires add steering load; see steering.
Speedometer
Ford's calibration procedure for 2011-2017 trucks shows that the speedometer is set for tire size. A different tire size makes it read wrong until it is recalibrated by a dealer or approved tool.
Where this comes from
- eCFR: 49 CFR 571.110, tire selection and rims (placard)
- U.S. Tire Manufacturers Association: Tire care and safety guide (PDF)
- NHTSA interpretation GF008935: tire placards after modification
- Ford 2011-2017 speedometer calibration procedure (copy filed with NHTSA)
Last reviewed 2026-10-19. General information only: Ford and your kit maker's instructions come first.