How this calculator works
Diameter change = new diameter − current diameter. Clearance change ≈ diameter change ÷ 2. Circumference = π × diameter. Sidewall height = (diameter − wheel diameter) ÷ 2. Theoretical actual speed = indicated speed × new diameter ÷ current diameter.
Worked example
A change from 25 × 8–12 to 27 × 9–12 adds 2 inches of nominal diameter, about 1 inch of ground clearance, and 1 inch of width. At an indicated 30 mph, the theoretical speed is 32.4 mph.
Common questions
Does a larger tire always fit?
No. Check approved wheel widths, rim diameter, offset, suspension clearance, steering clearance, and the tire maker’s measured dimensions.
Are the results stored or sent anywhere?
No. Your entries are used in your browser for this calculation. Calculator entries are not saved. Optional My Machine and checklist preferences are stored locally as described in our privacy policy.