225/55R18 tire size

A 225/55R18 tire is 27.7 inches (705 mm) tall and 8.9 inches (225 mm) wide, and fits a 18-inch wheel. In inch sizing it is about a 27.7x8.9R18.

Overall diameter27.74in705 mm

Drawn to scale from the nominal size.

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Size	Diameter	Difference	Width	Sidewall	Wheel
245/35R21	27.8″	+0.0%	9.6″	3.4″	21″
245/40R20	27.7″	−0.1%	9.6″	3.9″	20″
215/75R15	27.7″	−0.2%	8.5″	6.3″	15″
245/45R19	27.7″	−0.2%	9.6″	4.3″	19″
275/40R19	27.7″	−0.3%	10.8″	4.3″	19″
245/50R18	27.6″	−0.4%	9.6″	4.8″	18″
215/70R16	27.9″	+0.4%	8.5″	5.9″	16″
225/60R17	27.6″	−0.4%	8.9″	5.3″	17″
215/65R17	28.0″	+0.9%	8.5″	5.5″	17″
255/30R22	28.0″	+1.0%	10.0″	3.0″	22″
235/65R16	28.0″	+1.0%	9.3″	6.0″	16″
255/40R20	28.0″	+1.0%	10.0″	4.0″	20″

Within 3% of the original diameter is the range most tire shops treat as a direct swap. Check load index and clearance too.

225: Section width: 225 mm (8.9 in) from sidewall to sidewall.
55: Aspect ratio: the sidewall is 55% as tall as the tire is wide, 124 mm (4.9 in).
R: Radial construction, used by almost every modern tire.
18: Fits a 18-inch wheel.

225/55R18 is about 27.7 inches tall and 8.9 inches wide, on a 18-inch wheel, roughly a 27.7x8.9R18 in inch sizing.

Yes. At 27.7 inches it is sold and fitted as a 28-inch tire.

245/35R21, 245/40R20, 215/75R15 are the closest common sizes, all within 0.2% of its diameter, so the speedometer stays close to accurate.

225/60R18 is 3.2% taller, so at an indicated 60 mph you would really be doing 61.9 mph. Compare 225/55R18 and 225/60R18.

About 727 per mile (452 per km), from a 87.2-inch circumference.