235/40R18 tire size

A 235/40R18 tire is 25.4 inches (645 mm) tall and 9.3 inches (235 mm) wide, and fits a 18-inch wheel. In inch sizing it is about a 25.4x9.3R18.

Overall diameter25.40in645 mm

Drawn to scale from the nominal size.

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Size	Diameter	Difference	Width	Sidewall	Wheel
215/50R17	25.5″	+0.2%	8.5″	4.2″	17″
235/35R19	25.5″	+0.3%	9.3″	3.2″	19″
235/45R17	25.3″	−0.3%	9.3″	4.2″	17″
205/65R15	25.5″	+0.4%	8.1″	5.2″	15″
215/55R16	25.3″	−0.4%	8.5″	4.7″	16″
265/35R18	25.3″	−0.4%	10.4″	3.7″	18″
205/70R14	25.3″	−0.4%	8.1″	5.6″	14″
195/75R14	25.5″	+0.4%	7.7″	5.8″	14″
275/35R18	25.6″	+0.7%	10.8″	3.8″	18″
225/35R19	25.2″	−0.8%	8.9″	3.1″	19″
215/45R18	25.6″	+0.9%	8.5″	3.8″	18″
225/60R15	25.6″	+0.9%	8.9″	5.3″	15″

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

235: Section width: 235 mm (9.3 in) from sidewall to sidewall.
40: Aspect ratio: the sidewall is 40% as tall as the tire is wide, 94 mm (3.7 in).
R: Radial construction, used by almost every modern tire.
18: Fits a 18-inch wheel.

235/40R18 is about 25.4 inches tall and 9.3 inches wide, on a 18-inch wheel, roughly a 25.4x9.3R18 in inch sizing.

Nearly. It measures 25.4 inches, so it falls between 25 and 26 inches.

215/50R17, 235/35R19, 235/45R17 are the closest common sizes, all within 0.3% of its diameter, so the speedometer stays close to accurate.

255/40R18 is 2.5% taller, so at an indicated 60 mph you would really be doing 61.5 mph. Compare 235/40R18 and 255/40R18.

About 794 per mile (493 per km), from a 79.8-inch circumference.