255/45R18 tire size

A 255/45R18 tire is 27.0 inches (687 mm) tall and 10.0 inches (255 mm) wide, and fits a 18-inch wheel. In inch sizing it is about a 27.0x10.0R18.

Overall diameter27.04in687 mm

Drawn to scale from the nominal size.

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Size	Diameter	Difference	Width	Sidewall	Wheel
255/40R19	27.0″	−0.0%	10.0″	4.0″	19″
255/35R20	27.0″	−0.0%	10.0″	3.5″	20″
215/65R16	27.0″	−0.1%	8.5″	5.5″	16″
225/45R19	27.0″	−0.2%	8.9″	4.0″	19″
235/60R16	27.1″	+0.2%	9.3″	5.6″	16″
215/60R17	27.2″	+0.5%	8.5″	5.1″	17″
235/55R17	27.2″	+0.5%	9.3″	5.1″	17″
215/70R15	26.9″	−0.7%	8.5″	5.9″	15″
235/50R18	27.3″	+0.8%	9.3″	4.6″	18″
265/35R20	27.3″	+1.0%	10.4″	3.7″	20″
215/55R18	27.3″	+1.0%	8.5″	4.7″	18″
245/35R20	26.8″	−1.0%	9.6″	3.4″	20″

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.
45: Aspect ratio: the sidewall is 45% as tall as the tire is wide, 115 mm (4.5 in).
R: Radial construction, used by almost every modern tire.
18: Fits a 18-inch wheel.

255/45R18 is about 27.0 inches tall and 10.0 inches wide, on a 18-inch wheel, roughly a 27.0x10.0R18 in inch sizing.

Yes. At 27.0 inches it is sold and fitted as a 27-inch tire.

255/40R19, 255/35R20, 215/65R16 are the closest common sizes, all within 0.1% of its diameter, so the speedometer stays close to accurate.

235/55R18 is 4.2% taller, so at an indicated 60 mph you would really be doing 62.5 mph. Compare 255/45R18 and 235/55R18.

About 746 per mile (464 per km), from a 84.9-inch circumference.