Compare tire sizes for diameter, sidewall, circumference and speedometer error.
A metric tire like 225/45R17 means: 225 mm section width, 45% aspect ratio (sidewall = 45% of 225 = 101.25 mm), and 17-inch rim diameter. The overall diameter is twice the sidewall plus the rim. Circumference = π × diameter. Revolutions per mile = 63,360 in ÷ circumference. If you switch to a different size, your speedometer reads based on the old rev count; a larger tire makes the speedo under-read.
Overall diameter
Diameter (in) = 2 × (width × aspect/100) / 25.4 + rim diameter
Revolutions per mile
Revs/mile = 63,360 ÷ (π × diameter in inches)
The R stands for Radial construction — the internal cords run perpendicular to the direction of travel, from bead to bead. Virtually all modern passenger tires are radial.
In most countries, regulations permit speedometers to read up to ~10% high but never below actual speed. An error of 1–2% is essentially imperceptible. Beyond ±3% you may want to consider a speedometer recalibration or a closer size match.
A plus-one fitment (larger rim, lower profile tire) should maintain nearly the same overall diameter to avoid clearance issues. This calculator shows the diameter change so you can stay within the ±3% guideline.
225/45R17: 225 = section width in mm; 45 = aspect ratio (sidewall height as % of width); R = radial; 17 = rim diameter in inches. You can find this on the tire sidewall, usually preceded by the load index and speed rating.