How to Compute Between Standard & Metric Tire Sizes
The standard system defines a tire using the diameter (tire height), the width and the rim (wheel) size. All these dimensions are given in inches. 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.
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Instructions
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Standard to Metric
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1
Multiply the tire width (in inches) by 25.4 to calculate the width in millimeters. For example, if the tire size (in inches) is 31.6 x 12/16 (the width is 12 inches), then width = 12 x 25.4 = 305 mm (rounded).
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2
Subtract the rim (wheel) size from the tire diameter and then divide it by 2 to calculate the rim-to-tread distance. In our example, the distance is (31.6 - 16) / 2 = 7.8 inches.
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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 (7.8 / 12) x 100 = 65.
Thus, 31.6 x 12/15 converts to the metric system as 305/65/R16.
Metric to Standard
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4
Divide the tire width (in mm) by 25.4 to calculate the width in inches. Using the tire P265/65/R15 as an example, the width is 10.43 inches (265 / 25.4).
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5
Multiply the width (in inches) and the aspect ratio, and then divide the product by 100 to calculate the rim to tread distance. In our example, Distance = 10.43 x 65/100 = 6.78 inches.
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6
Multiply the rim to tread distance by 2 and add the wheel size to calculate the tire diameter. In our example, Diameter = (6.78 x 2) + 15 = 28.56. Thus, the tire 315/85/R15 converts to 28.56 x 10.43/15 inches.
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