The size of a tire is based on the tire being fully inflated and mounted on the specified width wheel, so measuring the actual size of a tire while it is mounted on a vehicle is often difficult. James Halderman photo

Reader has questions about tire size


Bob S. writes by email: “I am hoping you could clarify a couple things for me. I have a 245/45 tire on my car on which I am taking measurements. At 245, its tread width should be 9.6. I measure 8.5. Using the 245 to calculate the sidewall height, it should be 110 mm or 4.3 tall. Mine measures 3.5. I assume this measurement is taken from the ground surface to the outside edge of the rim. The numbers don’t seem to match. Are the measurements made for a tire not installed and inflated? Any explanation for the discrepancy?”


The width of the tire is not the width of the tread but instead is the “cross-sectional” width so this is why I think your measurements are not aligned to the size. The section width (245) is the maximum width of the tire (widest part of the sidewall) of the tire mounted on the nominal wheel width (as specified by the Tire and Rim Association) and inflated. The section height (45) is 45 percent of the section width. This includes the bead area which is below the wheel lip and is not visible/measurable when the tire is mounted. A 17-inch wheel is 17 inches. Diameter is measured where the tire bead sits, not the visible diameter of the wheel with a tire mounted.

Also, please note that tire size can vary by tire manufacturer. In fact, one “green” tire, as an uncured tire is called, can be made in many different sizes of tires, depending on which mold is used. In other words, while it appears to be very detailed as to the width and height that the tire should be, it is far from true for many tires. That is why it is recommended that the same size and brand of tire be used on the vehicle – especially all-wheel and four-wheel-drive vehicles.