Two size codes may produce nearly equal nominal outside diameters while differing substantially in width, sidewall height, rim requirement, load capability, speed capability, construction, or approved application. TireCalc intentionally refuses to label a pair compatible. Its purpose is to make one narrow geometry relationship visible before the user checks authoritative vehicle and tire information.
Nominal is not measured rolling size
The formula builds an ideal circle from the printed width, aspect ratio, and rim code. A real tire deforms at the contact patch and its loaded radius depends on pressure and load. Tread depth, wear, manufacturer design, measuring rim, and production tolerance also matter. Approximate circumference and revolutions therefore describe the formula, not a promise about one installed tire.
Vehicle clearance is three-dimensional
A diameter comparison cannot see rim width or offset, brake package, suspension travel, steering angle, fender shape, mud flaps, chains, axle spacing, or body modifications. Width change is displayed, but nominal width alone does not locate either sidewall inside the wheel well. The same pair can behave differently on two vehicles or on two wheels with different offsets.