Tire Diameter Calculator

What this number means

Every calculator on this site wants a true tire diameter, and the sidewall only promises a nominal one. A 285/70R17 computes to 32.7 inches; individual models vary ±0.5 inch around that, and a worn, loaded tire rolls shorter still. For purchase decisions the nominal number is fine. For speedometer correction, use the manufacturer's published revs-per-mile when they offer it.

What to change next

Measure what you own: chalk mark on the sidewall and the ground, roll one full revolution, measure the distance, divide by π. That rolling diameter — not the tape-measure height of an unloaded tire — is the number your gearing actually feels.

Worked example

315/70R17 (the JL Xtreme Recon and Bronco Sasquatch tire): 17 + 2 × (315 × 0.70 ÷ 25.4) = 34.4 inches, ~587 revs per mile. The flotation 35×12.50R17 next to it on the rack is a true 35 nominal — half an inch the charts treat as identical and your RPM won't.