205/40R18 tire size

A 205/40R18 tire is 24.46 inches (621.2 mm) tall, 8.07 in (205 mm) wide and fits a 18-inch wheel. In inch notation it is roughly a 24.5x8.1R18.

205/40R18 dimensions
Overall diameter	24.46 in	621.2 mm
Section width	8.07 in	205 mm
Sidewall height	3.23 in	82.0 mm
Circumference	76.83 in	1,952 mm
Revolutions	824.6 per mile	512.4 per km
Rim diameter	18 in	457.2 mm

Compare 205/40R18 with another size

Current size

New size

Speed in mphkm/h

Overall diameter difference

+0.08%

60 mph on the speedometer = 60.0 mph actual

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

Ground clearance: +0.3 mm (+0.01 in)
Revolutions: 825 → 824 per mile

205/40R18 235/35R18

Side-by-side dimensions

	Current	New	Difference	%
Diameter	24.46 in	24.48 in	+0.02 in	+0.08%
Diameter	621.2 mm	621.7 mm	+0.5 mm	+0.08%
Width	8.07 in	9.25 in	+1.18 in	+14.63%
Width	205.0 mm	235.0 mm	+30.0 mm	+14.63%
Sidewall	3.23 in	3.24 in	+0.01 in	+0.30%
Sidewall	82.0 mm	82.3 mm	+0.3 mm	+0.30%
Circumference	76.83 in	76.89 in	+0.06 in	+0.08%
Revs per mile	824.6	824.0	−0.7	−0.08%

Speedometer error

Speedometer shows	Actual speed	Difference
20 mph	20.0 mph	0.0 mph
30 mph	30.0 mph	0.0 mph
40 mph	40.0 mph	0.0 mph
50 mph	50.0 mph	0.0 mph
60 mph	60.0 mph	0.0 mph
70 mph	70.1 mph	+0.1 mph
80 mph	80.1 mph	+0.1 mph

Alternative sizes on the same 18-inch wheel

Sizes in use within ±3 % of the overall diameter of 205/40R18:

Plus and minus sizes for 205/40R18

Size	Wheel	Diameter	Difference
215/50R16	Minus 2 (16 in)	24.46 in	+0.03%
195/55R16	Minus 2 (16 in)	24.44 in	−0.05%
175/60R16	Minus 2 (16 in)	24.27 in	−0.77%
245/45R16	Minus 2 (16 in)	24.68 in	+0.92%
235/40R17	Minus 1 (17 in)	24.40 in	−0.23%
175/55R17	Minus 1 (17 in)	24.58 in	+0.50%
215/45R17	Minus 1 (17 in)	24.62 in	+0.66%
185/50R17	Minus 1 (17 in)	24.28 in	−0.71%
245/30R19	Plus 1 (19 in)	24.79 in	+1.35%
255/30R19	Plus 1 (19 in)	25.02 in	+2.32%

Vehicles approved with 205/40R18 tires

Models whose Swiss type approval lists 205/40R18 as a standard or optional size (1,506 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 18-inch sizes

Reading the 205/40R18 size

205 — section width in millimetres (8.07 in).
40 — aspect ratio: the sidewall is 40 % of the width, 82.0 mm.
R — radial construction (ZR on the sidewall means a high-speed rated radial; the dimensions are the same).
18 — fits a 18-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.