What does 315-30R21 mean?

Tire sizes are easy to understand once you learn how to read them.  On this page you'll find a color-coded explanation of the 315-30R21 tire code.

315

This number indicates that your tire has a width of 315 millimeters.

30

This number means that your tire has an aspect ratio of 30%.  In other words, your tire's sidewall height (from the edge of the rim to the tire's tread) is 30% of the width.  In this case, the sidewall height works out to be 94 millimeters.

R

This letter denotes how your tire was constructed.  Radial is the standard construction method for about 99% of all tires sold today.

RRadial
BBias Belt
DDiagonal
21

The tire size was designed to fit rims or wheels that are 21 inches in diameter.