225/40R18 tire size

A 225/40R18 tire is 25.1 inches (637 mm) tall and 8.9 inches (225 mm) wide, and fits a 18-inch wheel. In inch sizing it is about a 25.1x8.9R18.

Overall diameter25.09in637 mm

Drawn to scale from the nominal size.

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Size	Diameter	Difference	Width	Sidewall	Wheel
205/50R17	25.1″	−0.1%	8.1″	4.0″	17″
255/40R17	25.0″	−0.2%	10.0″	4.0″	17″
255/35R18	25.0″	−0.2%	10.0″	3.5″	18″
195/65R15	25.0″	−0.4%	7.7″	5.0″	15″
225/35R19	25.2″	+0.5%	8.9″	3.1″	19″
225/45R17	25.0″	−0.5%	8.9″	4.0″	17″
205/55R16	24.9″	−0.8%	8.1″	4.4″	16″
205/70R14	25.3″	+0.8%	8.1″	5.6″	14″
265/35R18	25.3″	+0.9%	10.4″	3.7″	18″
215/55R16	25.3″	+0.9%	8.5″	4.7″	16″
225/50R16	24.9″	−0.9%	8.9″	4.4″	16″
235/45R17	25.3″	+1.0%	9.3″	4.2″	17″

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

225: Section width: 225 mm (8.9 in) from sidewall to sidewall.
40: Aspect ratio: the sidewall is 40% as tall as the tire is wide, 90 mm (3.5 in).
R: Radial construction, used by almost every modern tire.
18: Fits a 18-inch wheel.

225/40R18 is about 25.1 inches tall and 8.9 inches wide, on a 18-inch wheel, roughly a 25.1x8.9R18 in inch sizing.

Yes. At 25.1 inches it is sold and fitted as a 25-inch tire.

205/50R17, 255/40R17, 255/35R18 are the closest common sizes, all within 0.2% of its diameter, so the speedometer stays close to accurate.

225/45R18 is 3.5% taller, so at an indicated 60 mph you would really be doing 62.1 mph. Compare 225/40R18 and 225/45R18.

About 804 per mile (500 per km), from a 78.8-inch circumference.