315/40R21 tire size

A 315/40R21 tire is 30.92 inches (785.4 mm) tall, 12.40 in (315 mm) wide and fits a 21-inch wheel. In inch notation it is roughly a 30.9x12.4R21 — a "31-inch" tire.

315/40R21 dimensions
Overall diameter	30.92 in	785.4 mm
Section width	12.40 in	315 mm
Sidewall height	4.96 in	126.0 mm
Circumference	97.14 in	2,467 mm
Revolutions	652.2 per mile	405.3 per km
Rim diameter	21 in	533.4 mm

Compare 315/40R21 with another size

Current size

New size

Speed in mphkm/h

Overall diameter difference

+0.57%

60 mph on the speedometer = 60.3 mph actual

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

Ground clearance: +2.3 mm (+0.09 in)
Revolutions: 652 → 649 per mile

315/40R21 285/45R21

Side-by-side dimensions

	Current	New	Difference	%
Diameter	30.92 in	31.10 in	+0.18 in	+0.57%
Diameter	785.4 mm	789.9 mm	+4.5 mm	+0.57%
Width	12.40 in	11.22 in	−1.18 in	−9.52%
Width	315.0 mm	285.0 mm	−30.0 mm	−9.52%
Sidewall	4.96 in	5.05 in	+0.09 in	+1.79%
Sidewall	126.0 mm	128.3 mm	+2.3 mm	+1.79%
Circumference	97.14 in	97.70 in	+0.56 in	+0.57%
Revs per mile	652.2	648.5	−3.7	−0.57%

Speedometer error

Speedometer shows	Actual speed	Difference
20 mph	20.1 mph	+0.1 mph
30 mph	30.2 mph	+0.2 mph
40 mph	40.2 mph	+0.2 mph
50 mph	50.3 mph	+0.3 mph
60 mph	60.3 mph	+0.3 mph
70 mph	70.4 mph	+0.4 mph
80 mph	80.5 mph	+0.5 mph

Alternative sizes on the same 21-inch wheel

Sizes in use within ±3 % of the overall diameter of 315/40R21:

Plus and minus sizes for 315/40R21

Size	Wheel	Diameter	Difference
285/50R20	Minus 1 (20 in)	31.22 in	+0.97%
295/45R20	Minus 1 (20 in)	30.45 in	−1.52%
285/45R20	Minus 1 (20 in)	30.10 in	−2.66%
325/35R22	Plus 1 (22 in)	30.96 in	+0.11%
285/40R22	Plus 1 (22 in)	30.98 in	+0.18%
315/35R22	Plus 1 (22 in)	30.68 in	−0.78%
295/40R22	Plus 1 (22 in)	31.29 in	+1.20%
285/35R23	Plus 2 (23 in)	30.85 in	−0.22%
295/35R23	Plus 2 (23 in)	31.13 in	+0.67%
325/30R23	Plus 2 (23 in)	30.68 in	−0.79%
315/30R23	Plus 2 (23 in)	30.44 in	−1.55%

Vehicles approved with 315/40R21 tires

Models whose Swiss type approval lists 315/40R21 as a standard or optional size (181 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 315/40R21 size

315 — section width in millimetres (12.40 in).
40 — aspect ratio: the sidewall is 40 % of the width, 126.0 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.