What does 235-40R18 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 235-40R18 tire code.

235

This number indicates that your tire has a width of 235 millimeters.

40

This number means that your tire has an aspect ratio of 40%.  In other words, your tire's sidewall height (from the edge of the rim to the tire's tread) is 40% 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
18

The tire size was designed to fit rims or wheels that are 18 inches in diameter.