Guide

How to read a motorcycle tire size

Most street motorcycle tires print a metric size on the sidewall. 180/55-17 is the example used throughout this site because it is a common sport and naked-bike rear size.

The first number is section width in millimeters. The second number is the aspect ratio: sidewall height as a percent of that width. The last number is the rim diameter in inches. A 180 mm tire with a 55 percent aspect has a 99 mm sidewall. A 17-inch rim is 431.8 mm. Overall diameter is 431.8 + 2 × 99 = 629.8 mm. Circumference is π × 629.8 mm, about 77.90 inches.

What the estimate leaves out

The stamp is a nominal size. Two tires with the same stamp can sit at different radii once they are mounted, loaded, and brought up to pressure. Wear lowers the radius over time. This site does not treat the estimate as an official specification for a VIN, and it does not set a pressure.

If you can wrap a tape through the centerline of the mounted rear tire, use that circumference on the speed-in-gear page. The checkbox labeled for a measured circumference bypasses the size geometry.

Changing size and the speedometer

If the instrument cluster was calibrated to the first size, a taller second size makes true road speed higher than the indicated figure. A shorter second size does the opposite. The tire size calculator reports that percent from circumference. It does not rewrite the cluster.