How we calculate tire dimensions

All dimensions come from the ISO metric size code printed on the sidewall, for example 275/60R20:

• 275 is the section width in millimetres.
• 60 is the aspect ratio: sidewall height as a percentage of section width.
• R means radial construction.
• 20 is the wheel (rim) diameter in inches.

Sidewall height = width × aspect ÷ 100. Overall diameter = rim + 2 × sidewall. Circumference = π × diameter. Revolutions per mile = 63,360 inches ÷ circumference. One inch = 25.4 mm.

Limits. These are nominal design dimensions. Real tires differ by brand, tread depth, load and rim width (a wider rim makes the same tire measure wider). Revolutions per mile from manufacturers are measured under load and are usually slightly higher than the calculated value. The 3% guideline for substitute sizes is a common rule of thumb, not a guarantee of fit. Always check load index, speed rating and clearance for your vehicle.

Which sizes appear on the site is based on how often each size is searched for in the United States.