Tire Size to Diameter Converter

When you need it

When converting a P-metric size into the diameter the gearing calculator wants.

Nominal, unloaded, and only approximate

This gives the nominal unloaded diameter from the printed size. Loaded rolling diameter is two to three percent smaller once the car is sitting on it.

Printed sizes are also nominal. Two tires with the same size on the sidewall from different manufacturers can differ by half an inch in real diameter, and drag radials vary more than street tires. If the number matters, measure the tire you own rather than trusting the label.

How to measure your actual rolling diameter

Put the car on the ground at its normal ride height and correct tire pressure. Mark the tire at the contact patch and mark the ground beside it. Roll the car forward until the tire mark comes back to the bottom, mark the ground again, and measure between the two marks.

That distance is the true rolling circumference. Divide it by pi to get the effective diameter, and use that in the gearing calculator. It accounts for load, pressure and the specific tire, which no printed size can.

Sizes that are not P-metric

Numeric drag tire sizes such as 28x10.50-15 already state the diameter directly, so there is nothing to convert — the first number is the diameter in inches. Older alpha-numeric sizes and flotation truck sizes follow their own conventions. This calculator handles the P-metric form only, which is the one where the diameter is hidden behind an aspect ratio.