255/40R18 tire size

A 255/40R18 tire is 26.0 inches (661 mm) tall and 10.0 inches (255 mm) wide, and fits a 18-inch wheel. In inch sizing it is about a 26.0x10.0R18.

Overall diameter26.03in661 mm

Drawn to scale from the nominal size.

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Size	Diameter	Difference	Width	Sidewall	Wheel
255/35R19	26.0″	−0.0%	10.0″	3.5″	19″
215/65R15	26.0″	−0.1%	8.5″	5.5″	15″
225/40R19	26.1″	+0.2%	8.9″	3.5″	19″
225/45R18	26.0″	−0.2%	8.9″	4.0″	18″
205/75R14	26.1″	+0.3%	8.1″	6.1″	14″
215/60R16	26.2″	+0.5%	8.5″	5.1″	16″
205/55R17	25.9″	−0.6%	8.1″	4.4″	17″
225/35R20	26.2″	+0.7%	8.9″	3.1″	20″
225/50R17	25.9″	−0.7%	8.9″	4.4″	17″
205/70R15	26.3″	+1.0%	8.1″	5.6″	15″
215/55R17	26.3″	+1.1%	8.5″	4.7″	17″
245/35R19	25.8″	−1.1%	9.6″	3.4″	19″

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.
40: Aspect ratio: the sidewall is 40% as tall as the tire is wide, 102 mm (4.0 in).
R: Radial construction, used by almost every modern tire.
18: Fits a 18-inch wheel.

255/40R18 is about 26.0 inches tall and 10.0 inches wide, on a 18-inch wheel, roughly a 26.0x10.0R18 in inch sizing.

Yes. At 26.0 inches it is sold and fitted as a 26-inch tire.

255/35R19, 215/65R15, 225/40R19 are the closest common sizes, all within 0.2% of its diameter, so the speedometer stays close to accurate.

255/45R18 is 3.9% taller, so at an indicated 60 mph you would really be doing 62.3 mph. Compare 255/40R18 and 255/45R18.

About 775 per mile (481 per km), from a 81.8-inch circumference.