When you're shopping for tires, you'll often see a number like 125 R on the sidewall of the tire. This number is called the tire's service description, and it provides important information about the tire. Here's a breakdown of what the numbers and letters in a typical service description mean:
The first number (125) is the width of the tire in millimeters. The second number (R) indicates that this is a radial tire. The last number (16) is the diameter of the wheel that this tire is designed to fit, in inches.
So, in this example, 125 R 16 means that the tire has a width of 125 mm and is a radial tire designed to fit a 16-inch wheel.