Street tires are rated using three numbers.
For an example using 225/70R15 or 225/70/15:
- The number 225 is the width of the tire in millimeters, measured from the inner sidewall to the outer sidewall.
- The number 70 is the aspect ratio, sometimes called the profile. This is the ratio of the sidewall height to the width. The sidewall height can be calculated by multiplying the ratio (70 or 0.70) by the width (225): 225 millimeters x 0.70 = 157.5 millimeters.
- The number 15, or R15, is the diameter of the wheel (or rim) given in inches. Since the width is a millimeter (metric system) measurement, it is a general practice to convert this measurement to the metric system: 15 x 25.4 = 381 millimeters. This is required when using the metric calculator.
- The final calculation would then be 157.5 mm x 2 (for two sidewalls = 315) + 381 mm (wheel diameter) = 696 mm. 696 mm / 25.4 (metric conversion to inches) yields 27.40" inches, or 27".