What Is the Meaning of Tire Size?

Tires come in all sorts of sizes. There are numbers and letters on the side of the tire, somewhere between the rim and the tread, that identify the specific size and type.

First Letter(s): Vehicle Rating

Most cars and light truck tires begin with the letter "P." That indicates the tire is for "passenger" vehicles. Some might have "LT" for light truck or "T" for temporary tires designed for short distances.

First Set of Numbers: Tire Width

This set of numbers is the width of the tire in millimeters. It is measured from sidewall to sidewall not tread width.

Second Set of Numbers: The Aspect Ratio

This is a ratio of the measurement from the inside rim of the tire to the top of the tread to the width of the tire, measured in millimeters. For a P21565, the width is 215 mm and measurement from the inner edge to the top of the tread is 65 percent of that, or 139.75 mm.

Next Letter: The Construction Type

Most are "R" for radial tires. Other letters are "B" for belted biased and "D" for diagonal biased.

Last Set of Numbers: Diameter

It is the diameter of your tire measured in inches, the same as the rim size.

References

• Coast Tire and Auto Service