Tire Size Calculator

A speedometer counts wheel revolutions and converts them to speed using the ORIGINAL tire's circumference. A larger tire covers MORE ground per revolution than the original, so at any given speedometer reading you're actually covering more distance than the gauge assumes -- your true speed and true odometer distance both end up higher than what's displayed.

Sidewall height is width times aspect ratio (converted from millimeters to inches), and overall diameter adds TWO sidewalls to the rim diameter -- one on top of the rim, one below it, since the tire wraps around the entire wheel. So Overall Diameter = Rim Diameter + 2 x Sidewall Height.

Plus-sizing pairs a wider tire and a larger rim with a proportionally LOWER aspect ratio, so the sidewall shrinks even as the tire gets wider -- since overall diameter is rim diameter plus twice the sidewall height, a bigger rim can be offset by a shorter sidewall (a lower aspect ratio applied to the wider width) to land on nearly the same total diameter, which is exactly why aspect ratio drops every time you plus-size to a bigger wheel.

Keeping overall diameter close to the original size (often called a "plus-size" fit, e.g., going to a wider tire on a bigger rim with a lower aspect ratio) keeps revolutions per mile close to stock, which keeps the speedometer error small -- a significantly larger or smaller diameter creates a speedometer and odometer inaccuracy that grows with the size difference.