Tire Size Reference

A tire marked 225/45R17 is not three measurements. It is one measurement, one percentage, and one measurement — and that middle number is why two tires with the same width can differ by three inches in height.

225 is the section width in millimeters, measured at the widest point of the sidewall — not the tread. 45 is the aspect ratio: sidewall height as a percentage of section width. So 225 × 0.45 = 101 mm of sidewall. R17 is a radial tire on a 17-inch wheel.

Overall diameter is the rim plus two sidewalls: 17 + 2 × 3.99 = 24.97 inches.

Section width is not tread width

This catches people fitting wider tires. The 225 measures the bulge of the sidewall, which is wider than the part touching the road. Clearance problems show up at the sidewall, at full steering lock and full suspension compression — not in the driveway.

Everything downstream reads off diameter

The speedometer counts wheel rotations. So does the odometer, the ABS, and on many vehicles the transmission shift schedule. Change the rolling diameter and all of them read wrong by the same percentage.

The industry works to ±3%. Inside that window nothing needs recalibrating. Outside it, several things do.

Flotation sizes are a different language

A tire marked 33×12.50R20 states the overall diameter directly — 33 inches — then the width in inches, then the rim. No arithmetic required, which is exactly why off-road tires use it.

What the numbers cannot tell you

Load index and speed rating come after the size, and they are not optional. A tire that fits geometrically but carries less than the vehicle's placard requires is the wrong tire, whatever the size says.