255/65R18 tire size

A 255/65R18 tire is 31.1 inches (789 mm) tall and 10.0 inches (255 mm) wide, and fits a 18-inch wheel. In inch sizing it is about a 31.1x10.0R18.

Overall diameter31.05in789 mm

Drawn to scale from the nominal size.

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Size	Diameter	Difference	Width	Sidewall	Wheel
255/70R17	31.1″	+0.0%	10.0″	7.0″	17″
255/55R20	31.0″	−0.0%	10.0″	5.5″	20″
275/65R17	31.1″	+0.1%	10.8″	7.0″	17″
31x10.50R15	31.0″	−0.2%	10.5″	8.0″	15″
285/50R20	31.2″	+0.5%	11.2″	5.6″	20″
235/75R17	30.9″	−0.6%	9.3″	6.9″	17″
275/45R21	30.7″	−1.0%	10.8″	4.9″	21″
275/40R22	30.7″	−1.3%	10.8″	4.3″	22″
245/75R17	31.5″	+1.3%	9.6″	7.2″	17″
265/70R16	30.6″	−1.4%	10.4″	7.3″	16″
265/65R17	30.6″	−1.6%	10.4″	6.8″	17″
265/65R18	31.6″	+1.6%	10.4″	6.8″	18″

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.
65: Aspect ratio: the sidewall is 65% as tall as the tire is wide, 166 mm (6.5 in).
R: Radial construction, used by almost every modern tire.
18: Fits a 18-inch wheel.

255/65R18 is about 31.1 inches tall and 10.0 inches wide, on a 18-inch wheel, roughly a 31.1x10.0R18 in inch sizing.

Yes. At 31.1 inches it is sold and fitted as a 31-inch tire.

255/70R17, 255/55R20, 275/65R17 are the closest common sizes, all within 0.1% of its diameter, so the speedometer stays close to accurate.

255/70R18 is 3.2% taller, so at an indicated 60 mph you would really be doing 61.9 mph. Compare 255/65R18 and 255/70R18.

About 650 per mile (404 per km), from a 97.6-inch circumference.