How to Compare Metric & Standard Tire Sizes

While the standard system used in the United States defines a tire by its height, width and rim size in inches, the metric system is slightly more complicated. Rather than defining measurements in inches, metric measurements are given in millimeters. What's more, the three-number metric system defines the width and rim size in metric units, but also includes an aspect ratio number in-between -- which is a percentage related to the distance between the rim to the tread and the width of the overall tire.

Instructions

Standard versus Metric

1

Check the sidewall of the tire to determine if you are dealing with a standard tire or a metric tire. With standard tires the numbers will be presented (for example) as 31.6 x 12/16, so the tire is 31.6 inches around on the outside diameter, 12 inches wide and 16 inches around the rim (the interior diameter) where it meets the wheel itself. In metric, those numbers would be represented as 305/65/R16. Compare accordingly.

2

Multiply the width of the tire by 25.4 to determine the dimension number in millimeters. Subtract the rim size from the tire diameter and divide it by 2 to determine the distance of the rim to the tread, and then divide the rim to tread distance by the width of the tire and multiply the result by 100 to generate the aspect ratio that the metric tires use. For example, a 31.6 x 12/15 tire converts over to 305/65/R16 when converted to metric measurements, with the middle number representing the aspect ratio.

3

Divide the width of the tire by 25.4 to transpose the metric width over to inches. Multiply the width of the tire in inches against the aspect ratio, and then divide the result by 100 to determine the rim to tread distance, and multiply that distance by 2 and add the size of the wheel to calculate the diameter of the actual tire. For example, a standard-measured tire would read 31.6 x 12/16 when converted over from the metric equivalent of 305/65/R16.