225/40R18 Tyre Size Calculator

Read a metric tyre size like 225/45R17 left to right: 225 is the section width in millimetres (the widest point of the inflated tyre), 45 is the aspect ratio — the sidewall height expressed as a percentage of that width, so the sidewall here is 45% of 225 mm ≈ 101 mm — R means radial construction (a D or B would mean diagonal/bias), and 17 is the wheel diameter in inches. From those three numbers every other figure follows: the overall diameter is the rim plus two sidewalls, the circumference is π times that diameter, and the revolutions per mile or kilometre are how many times the tyre turns to cover the distance — the number your speedometer and odometer are calibrated against. The inch or flotation designation such as 33x12.50R15 describes the same tyre differently: overall diameter 33″, section width 12.5″, on a 15″ rim. A metric size converts to inches exactly, but going the other way usually lands between standard sizes, so the honest answer is an exact conversion plus the nearest size actually sold. When changing size, the practical rule is to keep the overall diameter within about 3% of the original: beyond that the speedometer error grows, gearing and clearance change, and on many cars the readout and ABS/traction calibration drift. The load index and speed rating that follow a size (for example 91V) are ratings, not dimensions, so they don't affect these measurements — but always match or exceed the vehicle's specified load index, and never fit a lower speed rating than required.