Finding the Diameter of a Tire
You can use the numbers given in the tire specification to figure out the diameter of a tire. Remember, tires you put on your car should have the correct diameter or else the speedometer won't read correctly.
The diameter of a tire is the measure from one side to the other through center of the rim. It can be measured in any direction, as long as it passes through the center.
Since the tire rim measurements are still given in inches, we will use inches as the unit of measure.In order to figure out the diameter, in inches, we need to use this formula:
A x B ÷ 24.5 x 2 + R
Let's use the tire spec p185/65R15 to show how you fill in the numbers in the formula.
A is 185 mm, the width of the tire.
B is .65, the percent turned to a decimal. When you multiply A and B, you find the distance from the edge of the rim to the outside edge of the tire in millimeters. You divide this by 24.5 to change it to inches. You need to find 2 of these, so you multiply it by 2.
R is the diameter of the rim in inches.For this example, it is 15 inches.
To work out the diameter for this tire, you do this
185mm x .65 ÷ 24.5 x 2 + 15 inches = 24.82 inches
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