Tire Size Comparison

Comparing two tire sizes by diameter alone hides the change that most often causes the problem. Four numbers move when you change a tire, and they move independently.

Overall diameter drives the speedometer, the odometer, the ABS and the shift points. Section width decides whether it fits. Sidewall height decides how it rides and how easily you bend a wheel. Revolutions per mile is the same fact as diameter, stated the way manufacturers state it.

The speedometer lies in a specific direction

A taller tire covers more ground per rotation, so the speedometer reads low — you are going faster than the dash says. That is the direction that collects tickets. A shorter tire reads high, which is merely annoying.

Ground clearance changes by half

The tire grows at the top and at the bottom. Only the top half lifts the axle. This is where lift-and-tire discussions get muddled, because the full diameter change gets quoted as though all of it went into ride height.

Three percent is the line

Inside ±3%, the speedometer, odometer, ABS and stability control all keep working off their original calibration. Outside it, the speedometer error becomes a real number, the odometer under-reads — which matters at resale and for service intervals — and on many vehicles the transmission was calibrated to the original rolling diameter.

Geometry only, and that is a feature

There are no manufacturer fudge factors in these figures. Real molded sizes vary a few millimeters between brands, and a worn tire is up to ⅜″ smaller in diameter than a new one. Both of those are smaller than the differences worth comparing, but they are worth knowing before you argue with a tape measure.