How to Measure Tire Size?
4 Answers
You can check the sidewall of the tire for a series of alphanumeric characters to determine the tire size. Tire size meanings: Taking 225/65R17, 102V as an example, 225 represents a tire width of 225 millimeters, 65 indicates an aspect ratio of 65, R stands for radial tire, 17 denotes a tire size of 17 inches, 102 is the maximum load index, and V signifies that the tire's maximum speed is 240 kilometers per hour. Tire speed ratings: Tire speed ratings can go up to 300 kilometers per hour. Generally, ordinary passenger cars use U, H, and V-rated tires, with maximum speeds of 200, 210, and 240 kilometers per hour, respectively.
After repairing cars for over a decade, I've noticed many beginners miss key points when measuring tires. First, locate the stamped numbers on the tire sidewall like '215/55 R17'—these directly indicate the width, aspect ratio, and wheel size. If it's illegible, use a tape measure to gauge the widest part of the tread, ensuring the tire is on the ground under load for accuracy. For wheel size, measure the diameter of the inner metal rim; using calipers on the wheel hub's backplate protrusion is recommended. The most commonly overlooked step is checking the tire's production date—the last four digits of the DOT code, e.g., '3523', signify the 35th week of 2023. Also, insert a coin into the tread groove to measure depth; if it's below 1.6mm, replace the tire immediately as it critically affects braking distance.
Last year, I figured out some tricks when changing tires: prepare a tape measure and take photos with your phone. First, photograph the parameter label on the tire sidewall, then measure the tread width at its widest contact point—ensure the car is parked on level ground for accurate measurements. The aspect ratio is the sidewall height divided by the tread width; if measuring yourself, use a ruler vertically against the sidewall to measure the height, then divide by the tread width. For rim size, measure the inner diameter of the metal wheel rim, excluding any decorative outer covers. A reminder to pay attention to the final letter in the specifications, such as 91V, which indicates the load index and speed rating. I’ve seen cases where tires with a lower load rating caused wheel rim deformation.
Money-Saving Guide to DIY Tire Replacement: Accurate Measurement Without Professional Tools. The easiest way is to check the raised parameters on the tire sidewall during daytime. If no parameters are visible, use a tailor's soft tape to measure the inner circumference of the wheel hub, then divide by 3.14 to get the hub diameter. Measure tire width when cold, as thermal expansion can cause a 3mm difference. While at it, check for uneven wear—if the tread depth difference between inner and outer sides exceeds 1.5mm, a wheel alignment is needed. This basic check takes just three minutes monthly to prevent blowout risks.
