305/30R20 tire size

A 305/30R20 tire is 27.20 inches (691.0 mm) tall, 12.01 in (305 mm) wide and fits a 20-inch wheel. In inch notation it is roughly a 27.2x12.0R20.

305/30R20 dimensions
Overall diameter	27.20 in	691.0 mm
Section width	12.01 in	305 mm
Sidewall height	3.60 in	91.5 mm
Circumference	85.47 in	2,171 mm
Revolutions	741.3 per mile	460.7 per km
Rim diameter	20 in	508.0 mm

Compare 305/30R20 with another size

Current size

New size

Speed in mphkm/h

Overall diameter difference

−0.87%

60 mph on the speedometer = 59.5 mph actual

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

Ground clearance: −3.0 mm (−0.12 in)
Revolutions: 741 → 748 per mile

305/30R20 295/30R20

Side-by-side dimensions

	Current	New	Difference	%
Diameter	27.20 in	26.97 in	−0.24 in	−0.87%
Diameter	691.0 mm	685.0 mm	−6.0 mm	−0.87%
Width	12.01 in	11.61 in	−0.39 in	−3.28%
Width	305.0 mm	295.0 mm	−10.0 mm	−3.28%
Sidewall	3.60 in	3.48 in	−0.12 in	−3.28%
Sidewall	91.5 mm	88.5 mm	−3.0 mm	−3.28%
Circumference	85.47 in	84.72 in	−0.74 in	−0.87%
Revs per mile	741.3	747.8	+6.5	+0.88%

Speedometer error

Speedometer shows	Actual speed	Difference
20 mph	19.8 mph	−0.2 mph
30 mph	29.7 mph	−0.3 mph
40 mph	39.7 mph	−0.3 mph
50 mph	49.6 mph	−0.4 mph
60 mph	59.5 mph	−0.5 mph
70 mph	69.4 mph	−0.6 mph
80 mph	79.3 mph	−0.7 mph

Alternative sizes on the same 20-inch wheel

Sizes in use within ±3 % of the overall diameter of 305/30R20:

Plus and minus sizes for 305/30R20

Size	Wheel	Diameter	Difference
285/40R18	Minus 2 (18 in)	26.98 in	−0.84%
275/45R18	Minus 2 (18 in)	27.74 in	+1.98%
275/40R18	Minus 2 (18 in)	26.66 in	−2.00%
295/35R19	Minus 1 (19 in)	27.13 in	−0.27%
305/35R19	Minus 1 (19 in)	27.41 in	+0.74%
285/35R19	Minus 1 (19 in)	26.85 in	−1.29%
275/40R19	Minus 1 (19 in)	27.66 in	+1.68%
275/30R21	Plus 1 (21 in)	27.50 in	+1.07%
295/25R21	Plus 1 (21 in)	26.81 in	−1.46%
285/30R21	Plus 1 (21 in)	27.73 in	+1.94%
345/25R21	Plus 1 (21 in)	27.79 in	+2.16%

Vehicles approved with 305/30R20 tires

Models whose Swiss type approval lists 305/30R20 as a standard or optional size (542 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 20-inch sizes

Reading the 305/30R20 size

305 — section width in millimetres (12.01 in).
30 — aspect ratio: the sidewall is 30 % of the width, 91.5 mm.
R — radial construction (ZR on the sidewall means a high-speed rated radial; the dimensions are the same).
20 — fits a 20-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.