In more detail
A full marking such as 225/45R17 91W reads: 225 mm section width, a sidewall 45% as tall as the width (101.25 mm), R for radial construction, a 17-inch rim, load index 91 (615 kg per tyre) and speed rating W (270 km/h). US sizes may start with P (passenger) or LT (light truck). Overall diameter in inches is rim + 2 × width × aspect / 100 / 25.4.
Why it matters
Changing tyre size changes the overall diameter, which shifts speedometer and odometer readings, affects stability-control calibration and can cause rubbing. A common guideline keeps the new diameter within about 3% of the original. The load index and speed rating should be at least those given on the vehicle's tyre placard, usually inside the driver's door.
Example
225/45R17: sidewall 225 × 0.45 = 101.25 mm
Diameter: 17 + 2 × 101.25 / 25.4 = 24.97 in (634.3 mm), 807.6 revolutions per mile
245/40R18: 18 + 2 × 98 / 25.4 = 25.72 in → 2.98 % larger
Speedometer reading 60 mph → actually 61.8 mph