Tire Size Calculator

Enter your original tire size and a candidate size to see overall diameter, sidewall, revolutions per mile, and speedometer error. Useful for plus-sizing, lift-kit fitment, or replacing OEM tires.

AOriginal size

BNew size

Tire Size Visualizer

OriginalNew

225/45R17

Sidewall101.3 mm

Overall diameter634.3 mm (24.97")

Revs / mile808

245/40R18

Sidewall98.0 mm

Overall diameter653.2 mm (25.72")

Revs / mile784

Difference (new − original)

Overall diameter+18.9 mm+2.98%

Sidewall-3.3 mm

Section width+20 mm

Ride height (axle)+9.5 mm

At indicated 60 mph61.79 mphactual speed

Speedo error

How tire-size math works

A tire size like 275/35R19 tells you three things: the section width in millimeters (275), the aspect ratio as a percentage of the section width (35%), and the rim diameter in inches (19). From those three numbers everything else is derived.

Sidewall

Sidewall height = width × aspect ratio / 100. For 275/35R19 that's 275 × 0.35 = 96.25 mm.

Overall diameter

Overall diameter = (sidewall × 2) + (rim × 25.4). The 25.4 converts inches to millimeters.

Speedometer error

Your speedometer reads from wheel rotations. If your new tire is bigger, each rotation covers more ground, so your actual speed is higher than indicated. The percentage error equals the percentage change in overall diameter — and most manufacturers want it kept under ±3%.