Tire size comparison

Enter a metric size such as 225/45R17: width 225 mm, sidewall height 45% of width, rim 17 inches. Spaces, lowercase r and a dash instead of R are accepted, as is one decimal digit in the rim diameter. Load/speed suffixes and inch-only or ETRTO bicycle formats are not parsed. Leaving the replacement empty gives one tire’s dimensions.

Sidewall = width × aspect / 100; diameter = 2 × sidewall + rim × 25.4; circumference = π × diameter. The illustration shows both outer tire and rim on one scale. It does not show tread width or a complete wheel installation. Radius change is half the diameter difference, a geometric change only.

The estimated speed assumes the original tire gave an exactly correct speedometer reading and the same wheel rotation signal is used after replacement: estimated road speed = indicated speed × new circumference / original circumference. Real factory offsets, sensor calibration, tread wear and loaded rolling circumference are not modeled. The expandable RPM comparison is a separate scenario at the same ROAD speed for both tires: RPM = road speed × 1,000,000 / (60 × circumference in mm). At the same speedometer reading, the wheel signal/RPM would instead be unchanged. Clear the speed input to omit both speed calculations.

Dimensions are nominal approximations. Pressure, rim width, load and the tire itself affect actual rollout. Width, clearance, load/speed rating, brake clearance and vehicle approval must be assessed separately; no percentage threshold here declares a replacement compatible. Sources: Continental: metric tire codes and Schwalbe: actual tire dimensions.