There are three numbers on tire sizes, which represent the width of the tire, the height of the tire, and the diameter of the tire. The width of a tire is measured in millimeters, which is a measurement that indicates the width of the tire from side to side. A height-to-width ratio is used when measuring the height of a building. The diameter of the tire is measured in inches and it is the measurement that tells you what size rim or wheel the tire will fit on. In the example above, the height of 225, the width of 225, and the diameter of 17 would make 225/50/17 a tire with a diameter of 17 and a width of 225.