275/45R21 tire size

A 275/45R21 tire is 30.7 inches (781 mm) tall and 10.8 inches (275 mm) wide, and fits a 21-inch wheel. In inch sizing it is about a 30.7x10.8R21.

Overall diameter30.74in781 mm

Drawn to scale from the nominal size.

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Size	Diameter	Difference	Width	Sidewall	Wheel
275/40R22	30.7″	−0.3%	10.8″	4.3″	22″
235/75R17	30.9″	+0.4%	9.3″	6.9″	17″
265/70R16	30.6″	−0.4%	10.4″	7.3″	16″
265/65R17	30.6″	−0.6%	10.4″	6.8″	17″
265/60R18	30.5″	−0.7%	10.4″	6.3″	18″
245/70R17	30.5″	−0.8%	9.6″	6.8″	17″
31x10.50R15	31.0″	+0.8%	10.5″	8.0″	15″
245/75R16	30.5″	−0.9%	9.6″	7.2″	16″
255/55R20	31.0″	+1.0%	10.0″	5.5″	20″
255/65R18	31.1″	+1.0%	10.0″	6.5″	18″
255/70R17	31.1″	+1.0%	10.0″	7.0″	17″
265/50R20	30.4″	−1.0%	10.4″	5.2″	20″

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

275: Section width: 275 mm (10.8 in) from sidewall to sidewall.
45: Aspect ratio: the sidewall is 45% as tall as the tire is wide, 124 mm (4.9 in).
R: Radial construction, used by almost every modern tire.
21: Fits a 21-inch wheel.

275/45R21 is about 30.7 inches tall and 10.8 inches wide, on a 21-inch wheel, roughly a 30.7x10.8R21 in inch sizing.

Yes. At 30.7 inches it is sold and fitted as a 31-inch tire.

275/40R22, 235/75R17, 265/70R16 are the closest common sizes, all within 0.4% of its diameter, so the speedometer stays close to accurate.

About 656 per mile (408 per km), from a 96.6-inch circumference.