Tire diameter formula with known metric tire size.

Some have posted on tire size swaps while seeking a specific tire diameter. Here's the formula for tire diameter.

Tire size is, of course, listed as "SW/AR-RD", where SW is section width (the width of the tire), AR is aspect ratio (the height of the sidewall of the tire expressed as a percentage of width), and RD is the diameter of the rim the tire is designed to be mounted on. E.G., 235/45-16 is a 235 millimeter wide tire with a sidewall height 45% of the width, designed to be mounted on a 16 inch rim.

Overall tire diameter is as follows:

TD = (SW x AR x 2) / 25.4 + RD

The term in parentheses is the height of both sidewalls and dividing by 25.4 converts the units from millimeters into inches for addition to the rim diameter. E.G., (235 x .45 x 2)/25.4 + 16 gives an overall diameter of the above mentioned tire of 24.3 inches.

That is nice and sorta helpful, but as anyone who has worked in a tire store can tell you, 2 different brands of tire the same size can be different heights.

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That is nice and sorta helpful, but as anyone who has worked in a tire store can tell you, 2 different brands of tire the same size can be different heights.

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You mean same brand tires can have different heights