How to read a tire size

Every tire has its size molded into the sidewall. A common example looks like 235/40 ZR18. Each part of that code tells you something different.

The first number, 235, is the section width in millimeters. It is the width of the tire from sidewall to sidewall when mounted on its measuring rim and inflated.

The second number, 40, is the aspect ratio. It is the sidewall height as a percentage of the section width. Here the sidewall is 40 percent of 235 mm, about 94 mm.

The letter R means radial construction, which is how almost all modern car tires are built. A Z before the R, as in ZR, is an older way of marking a tire rated for high speeds. For fitment, a ZR18 and an R18 tire of the same width and aspect ratio share the same dimensions.

The last number, 18, is the rim diameter in inches. The tire only fits a wheel of that diameter.

You may also see a load index and speed rating after the size, such as 94W. Those describe the maximum load and speed the manufacturer rates the tire for.

To find your size, read it off the tire currently on the car or check the placard on the driver's door jamb, then use the size search on our tire collection.