The difference between these two tires is in their aspect ratio, a comparison between the tire's height and width. The tire's aspect ratio is the tire's sidewall height expressed as a percentage of its width. In a 275 70 18 tire, the sidewall is 275 millimeters x 70 percent or 192.5 millimeters. In the second tire, the sidewall height is 275 millimeters x 65 percent or 178.75 millimeters.
A tire's aspect ratio affects the handling of a vehicle. Tires with a lower aspect ratio have shorter, stiffer sidewalls, improving their stability when cornering. High performance tires generally have an aspect ratio of less than 50.
Tire sizes include several letters and numbers. The first letter or series of two letters designates the type of vehicle on which the tire is designed for use. P is for passenger vehicles, LT is for light trucks, and T is for temporary use tires or spares. The next series of three digits indicates the width of the tire in millimeters. The series of two digits is the aspect ratio. An R after the aspect ratio indicates a radial tire. The final series of digits in the tire size are the wheel size. The series of numbers and letters that follow the tire size indicate the tire's load range and speed rating.