Measure the Diameter first. Measure your tire across the face from one side to the other at the point where the front of the tire brakes onto the tread. Move your tape up and down till you have achieved the most significant distance. This will be the Diameter of the tire.
Measure Across the Tread second. Stand on the side of the tire and measure the width of the tread section of the tire. This will give you the tread with of the tire.
You can also measure the circumference of the tire. Third. To do this, stand in front of the face of the tire and hold the beginning of the tape measure on the top of the tire, pulling it around the edge of the tread surface till it meets up with the starting place of the tape. That length is equal to the circumference of the tire.
These are the measurements to pick out your proper size for a cover. Many tire cover charts are available online and on most package backs in retail stores to help you choose. A link to the Tire-Covers Size chart is here.
Author Stephen C Foster Sr.