Tire Tech

Understanding your tires


Tire Size

Tire sizes for passenger cars are generally reported in the format shown in the picture, “P215/65R15.” The P indicates that the tire is intended for a passenger car. The 215 indicates the width of the tire in millimeters, whereas the 65 indicates the ratio of the height of the sidewall to the width of the tire in millimeters. The R indicates that the tire is a radial and the 15 indicates the diameter of the wheel in inches.

Load Index/Speed Symbol

The load index corresponds with the load capacity of the tire. The higher the load index, the more weight the tire can support. The speed symbol corresponds with the maximum safe operating speed of the tire in MPH. Most passenger tires have a minimum speed rating of Q (up to 100 MPH) whereas high performance tires with a Z rating can withstand speeds of 149 MPH and up. The pictured tire would only have a speed rating of about 60 MPH and thus does not represent a typical passenger car tire.

DOT Number

The DOT number imprinted on the tire indicates that the tire meets all DOT standards to be an approved tire for sale in the US. The most important numbers for the consumer are the last four, “2910” in the image to the left. The “29” indicates that the tire was manufactured in the 29th week of 2010, which is indicated by the “10” coming after the “29.” Make sure when you get new tires that they aren’t 2 or 3 years old as the service life of a tire is only about 6 years depending on the manufacturer.