255/55R18 tire size

A 255/55R18 tire is 29.0 inches (738 mm) tall and 10.0 inches (255 mm) wide, and fits a 18-inch wheel. In inch sizing it is about a 29.0x10.0R18.

Overall diameter29.04in738 mm

Drawn to scale from the nominal size.

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Size	Diameter	Difference	Width	Sidewall	Wheel
255/50R19	29.0″	−0.0%	10.0″	5.0″	19″
255/65R16	29.1″	+0.0%	10.0″	6.5″	16″
255/45R20	29.0″	−0.0%	10.0″	4.5″	20″
255/70R15	29.1″	+0.0%	10.0″	7.0″	15″
255/40R21	29.0″	−0.0%	10.0″	4.0″	21″
235/65R17	29.0″	−0.1%	9.3″	6.0″	17″
285/45R19	29.1″	+0.2%	11.2″	5.0″	19″
235/60R18	29.1″	+0.2%	9.3″	5.6″	18″
295/35R21	29.1″	+0.3%	11.6″	4.1″	21″
235/70R16	29.0″	−0.3%	9.3″	6.5″	16″
235/55R19	29.2″	+0.5%	9.3″	5.1″	19″
235/75R15	28.9″	−0.6%	9.3″	6.9″	15″

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

255: Section width: 255 mm (10.0 in) from sidewall to sidewall.
55: Aspect ratio: the sidewall is 55% as tall as the tire is wide, 140 mm (5.5 in).
R: Radial construction, used by almost every modern tire.
18: Fits a 18-inch wheel.

255/55R18 is about 29.0 inches tall and 10.0 inches wide, on a 18-inch wheel, roughly a 29.0x10.0R18 in inch sizing.

Yes. At 29.0 inches it is sold and fitted as a 29-inch tire.

255/50R19, 255/65R16, 255/45R20 are the closest common sizes, all within 0.0% of its diameter, so the speedometer stays close to accurate.

235/65R18 is 3.4% taller, so at an indicated 60 mph you would really be doing 62.0 mph. Compare 255/55R18 and 235/65R18.

About 694 per mile (431 per km), from a 91.2-inch circumference.