If the tires on your car don't fit perfectly, your car won't ride smoothly and it can be dangerous. It's really not necessary to measure your car tires. All the information is on the tire, albeit in code. You just have to know how to read the code. Here's a quick explanation. Look at the side of the tire and you will see a jumble of letters and numbers. The first letter will tell you the class of tire. P is for passenger, LT is for light truck and T is for temporary spare. The numbers following this letter indicate the width of the tire in millimeters (an internationally accepted measure). Then there is slash and a second set of numbers, which represent the ratio of the tire's height to its width. That is followed by a letter R for radial, and then another set of numbers which denotes the diameter of the wheel in inches [source: Dunlop Tires]. We will now explain how to take measurements of a tire using a tape measure.

  1. Make sure the tire is completely inflated and is mounted on the vehicle
  2. Measure the width of the tread of the tire. This is the distance from the tread's inner edge to its outer edge.
  3. Measure the diameter of the tire. This is the outside width of the tire. You place the tape measure from one end of the tire to the other across the center.
  4. Measure the height of the tire. This is the vertical distance from where the tire meets the wheel to the top of the tread [source: Tire Chain].