255/40R21 tire size

A 255/40R21 tire is 29.03 inches (737.4 mm) tall, 10.04 in (255 mm) wide and fits a 21-inch wheel. In inch notation it is roughly a 29.0x10.0R21 — a "29-inch" tire.

255/40R21 dimensions
Overall diameter	29.03 in	737.4 mm
Section width	10.04 in	255 mm
Sidewall height	4.02 in	102.0 mm
Circumference	91.21 in	2,317 mm
Revolutions	694.7 per mile	431.7 per km
Rim diameter	21 in	533.4 mm

Compare 255/40R21 with another size

Current size

New size

Speed in mphkm/h

Overall diameter difference

+0.34%

60 mph on the speedometer = 60.2 mph actual

Within the ±3 % diameter range most tire fitters accept.

Ground clearance: +1.3 mm (+0.05 in)
Revolutions: 695 → 692 per mile

255/40R21 295/35R21

Side-by-side dimensions

	Current	New	Difference	%
Diameter	29.03 in	29.13 in	+0.10 in	+0.34%
Diameter	737.4 mm	739.9 mm	+2.5 mm	+0.34%
Width	10.04 in	11.61 in	+1.57 in	+15.69%
Width	255.0 mm	295.0 mm	+40.0 mm	+15.69%
Sidewall	4.02 in	4.06 in	+0.05 in	+1.23%
Sidewall	102.0 mm	103.3 mm	+1.3 mm	+1.23%
Circumference	91.21 in	91.51 in	+0.31 in	+0.34%
Revs per mile	694.7	692.4	−2.3	−0.34%

Speedometer error

Speedometer shows	Actual speed	Difference
20 mph	20.1 mph	+0.1 mph
30 mph	30.1 mph	+0.1 mph
40 mph	40.1 mph	+0.1 mph
50 mph	50.2 mph	+0.2 mph
60 mph	60.2 mph	+0.2 mph
70 mph	70.2 mph	+0.2 mph
80 mph	80.3 mph	+0.3 mph

Alternative sizes on the same 21-inch wheel

Sizes in use within ±3 % of the overall diameter of 255/40R21:

Plus and minus sizes for 255/40R21

Size	Wheel	Diameter	Difference
255/50R19	Minus 2 (19 in)	29.04 in	+0.03%
285/45R19	Minus 2 (19 in)	29.10 in	+0.23%
235/55R19	Minus 2 (19 in)	29.18 in	+0.50%
275/45R19	Minus 2 (19 in)	28.74 in	−0.99%
255/45R20	Minus 1 (20 in)	29.04 in	+0.01%
285/40R20	Minus 1 (20 in)	28.98 in	−0.19%
235/50R20	Minus 1 (20 in)	29.25 in	+0.76%
295/40R20	Minus 1 (20 in)	29.29 in	+0.90%
255/35R22	Plus 1 (22 in)	29.03 in	−0.01%
295/30R22	Plus 1 (22 in)	28.97 in	−0.22%
305/30R22	Plus 1 (22 in)	29.20 in	+0.60%
265/35R22	Plus 1 (22 in)	29.30 in	+0.94%

Vehicles approved with 255/40R21 tires

Models whose Swiss type approval lists 255/40R21 as a standard or optional size (1,174 approvals in total). Always check your own vehicle's placard or papers.

Source: Swiss Federal Roads Office (FEDRO/ASTRA), open government data. Original data (downloaded 2026-09-29). Data processed by us; errors in processing are ours, not the publisher's.

Nearby 21-inch sizes

Reading the 255/40R21 size

255 — section width in millimetres (10.04 in).
40 — aspect ratio: the sidewall is 40 % of the width, 102.0 mm.
R — radial construction (ZR on the sidewall means a high-speed rated radial; the dimensions are the same).
21 — fits a 21-inch diameter wheel.

The load index and speed rating (for example 91V) follow the size on the sidewall. They must meet at least the values on your vehicle's tire placard.