What does 255-70R18 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 255-70R18 tire code.
This number indicates that your tire has a width of 255 millimeters.
This number means that your tire has an aspect ratio of 70%. In other words, your tire's sidewall height (from the edge of the rim to the tire's tread) is 70% of the width. In this case, the sidewall height works out to be 178 millimeters.
The tire size was designed to fit rims or wheels that are 18 inches in diameter.