A tire's section width, also known as cross section width, is the measurement of the tire's linear width from sidewall to sidewall (excluding any raised letters, ornamentation, or protective ribs).
About Section Width
It's important to note that section width is not the overall width of the tire and this measurement is done on an inflated tire (without any load applied) after it has been mounted on a proper wheel assembly. Factors such as air pressure and load weight can affect the operating section width of a tire. Section width is used for sizing purposes only. A tire's section width is also heavily influenced by the width of the wheel upon which the tire is mounted. For example, the section width of a tire mounted on a narrow rim would be "narrower" than if the same tire were mounted on a wider rim. Because of this, there is a specific rim width range designated for every tire size. Tires should only be mounted on wheels within the tire manufacturer’s predetermined rim width range.
Section Width on Metric Tires
If you are driving on a metric tire, the sidewall code will look something like this: P215/65/R17 99H. The first number set in the code indicates the section width of your tire. In this case, the section width is 215 millimeters wide.
Section Width on High Flotation Tires
If your tires are high flotation tires, the sidewall code will look something like this: 31X10.50/R-15. In this scenario, the number set that represents the section width is not the first, but the second set, 10.50. High flotation tires are measured in inches. So the section width in this example is 10.5 inches.