[header] How to measure tires [title] Check the tire's sidewall to read its wheel diameter. [step] The wheel diameter is often, though not always, printed on the tire's sidewall. You will be able to spot the diameter number by looking for the number followed after a capitalized " r. " [substeps] The tire diameter may be printed in inches or meters, depending on your country. 
If the wheel diameter is 15 inches (38 cm), for example, the diameter will read as " r15 " on the number string. [title] Lay the tire down on its side to measure wheel diameter manually.