How to Measure Tires

When buying new tires, measure their width when they are on your vehicle to make sure they are properly inflated and won't roll unevenly on your wheels. Measuring your tires isn't an exercise in higher math, and you can easily deduce the information you need.

Things You'll Need

• Tape measure

Instructions

Understand the Code

1. Check the first letter on the code on the side of the tire. It tells you what type of vehicle it is for ("P" for passenger vehicles, "LT" for light truck and "T" for a spare tire).

2. Look at the first three numbers on the code of the tire, the ones that come after the first letter. They tell you its width in mm if you measure from sidewall to sidewall.

3. Read the number after the slash on the tire's code. It tells you the tire's height (from the bead or the top of the rim to the top of the tread), expressed as a percentage of the width. So "75" would mean a tire height measuring 75 percent of the width, in mm.

4. Check the first letter after the slash in the code of the tires. It indicates the tire's construction: "R" for radial, "D" for diagonal bias and "B" for bias belted. The last two are usually older tires, which you don't see often on modern cars.

5. Examine the last numbers on the tire code (the ones after the letter indicating its construction). These state the tire's wheel rim diameter in inches, which determines what size of rim the tires will fit on.

Take the Measurements

1. Measure the width of the tire to make sure it conforms to its stated width. If the numbers are off, it is probably under- or over-inflated.

2. Measure the diameter of the wheel rim from top to bottom, without including the tire in the measurement.

3. Determine the tire's height by multiplying its stated width by the percentage listed on its code (for example, 240 times 75 percent equals 180 mm or approximately 7.086 inches).

4. Multiply the tire's height by 2 and add the result to the diameter of the wheel rim. This is the overall diameter of the wheel.

5. Measure the tire from the ground to the top of the tread to make sure it conforms to your calculations.

Tips & Warnings

• When you measure a tire, remember that the numbers are usually for the tire when unloaded. When it is attached to the vehicle, weight is put on it and its parameters may shift to support the load (specifically, its diameter will decrease).