255/40R21 tire size

A 255/40R21 tire is 29.0 inches (737 mm) tall and 10.0 inches (255 mm) wide, and fits a 21-inch wheel. In inch sizing it is about a 29.0x10.0R21.

Overall diameter29.03in737 mm

Drawn to scale from the nominal size.

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Size	Diameter	Difference	Width	Sidewall	Wheel
235/65R17	29.0″	−0.0%	9.3″	6.0″	17″
255/45R20	29.0″	+0.0%	10.0″	4.5″	20″
255/50R19	29.0″	+0.0%	10.0″	5.0″	19″
255/55R18	29.0″	+0.0%	10.0″	5.5″	18″
255/65R16	29.1″	+0.1%	10.0″	6.5″	16″
255/70R15	29.1″	+0.1%	10.0″	7.0″	15″
285/45R19	29.1″	+0.2%	11.2″	5.0″	19″
235/60R18	29.1″	+0.2%	9.3″	5.6″	18″
235/70R16	29.0″	−0.3%	9.3″	6.5″	16″
295/35R21	29.1″	+0.3%	11.6″	4.1″	21″
235/55R19	29.2″	+0.5%	9.3″	5.1″	19″
235/75R15	28.9″	−0.5%	9.3″	6.9″	15″

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.
21: Fits a 21-inch wheel.

255/40R21 is about 29.0 inches tall and 10.0 inches wide, on a 21-inch wheel, roughly a 29.0x10.0R21 in inch sizing.

Yes. At 29.0 inches it is sold and fitted as a 29-inch tire.

235/65R17, 255/45R20, 255/50R19 are the closest common sizes, all within 0.0% of its diameter, so the speedometer stays close to accurate.

275/45R21 is 5.9% taller, so at an indicated 60 mph you would really be doing 63.5 mph. Compare 255/40R21 and 275/45R21.

About 695 per mile (432 per km), from a 91.2-inch circumference.