How to Perform an Inch to Tire Conversion

There are two systems of the tire description. The standard system defines dimensions of a tire (in inches) as its diameter (tire height) and width, with the third parameter being a rim (wheel) size. The metric system is less straightforward and describes a tire with the three numbers, for instance "P265/65/R15." The first number is the tire width in millimeters. The second one is an aspect ratio (in percent) that is a ratio of the distance from the rim to the tread to the width of the tire. The third parameter ("R15") is the same as in the standard system. "P" stands for a passenger car.

Instructions

1

Multiply the tire width (in inches) by 25.4 to calculate the width in millimeters. For example, the tire size (in inches) is given as 36.1 x 12.4/15. The tire width is 12.4 in. That is equal to 12.4 x 25.4 = 315 mm.

2

Subtract the rim (wheel) size from the tire diameter and then divide by two to calculate the rim-to-tread distance. In our example, the distance is (36.1 - 15)/ 2 = 10.55 inches.

3

Divide the rim-to-tread distance (Step 2) by the tire width (in inches) and then multiply by 100 to calculate the aspect ratio. In our example, the aspect ratio is (10.55 / 12.4) x 100 = 85.
Thus, the tire size of 36.1 x 12.4/15 inches converts to the metric system as 315/85/R15.