225/40R18 tire size

A 225/40R18 tire is 25.09 inches (637.2 mm) tall, 8.86 in (225 mm) wide and fits a 18-inch wheel. In inch notation it is roughly a 25.1x8.9R18.

225/40R18 dimensions
Overall diameter	25.09 in	637.2 mm
Section width	8.86 in	225 mm
Sidewall height	3.54 in	90.0 mm
Circumference	78.81 in	2,002 mm
Revolutions	803.9 per mile	499.5 per km
Rim diameter	18 in	457.2 mm

Compare 225/40R18 with another size

Current size

New size

Speed in mphkm/h

Overall diameter difference

−0.24%

60 mph on the speedometer = 59.9 mph actual

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

Ground clearance: −0.8 mm (−0.03 in)
Revolutions: 804 → 806 per mile

225/40R18 255/35R18

Side-by-side dimensions

	Current	New	Difference	%
Diameter	25.09 in	25.03 in	−0.06 in	−0.24%
Diameter	637.2 mm	635.7 mm	−1.5 mm	−0.24%
Width	8.86 in	10.04 in	+1.18 in	+13.33%
Width	225.0 mm	255.0 mm	+30.0 mm	+13.33%
Sidewall	3.54 in	3.51 in	−0.03 in	−0.83%
Sidewall	90.0 mm	89.3 mm	−0.8 mm	−0.83%
Circumference	78.81 in	78.63 in	−0.19 in	−0.24%
Revs per mile	803.9	805.8	+1.9	+0.24%

Speedometer error

Speedometer shows	Actual speed	Difference
20 mph	20.0 mph	0.0 mph
30 mph	29.9 mph	−0.1 mph
40 mph	39.9 mph	−0.1 mph
50 mph	49.9 mph	−0.1 mph
60 mph	59.9 mph	−0.1 mph
70 mph	69.8 mph	−0.2 mph
80 mph	79.8 mph	−0.2 mph

Alternative sizes on the same 18-inch wheel

Sizes in use within ±3 % of the overall diameter of 225/40R18:

Plus and minus sizes for 225/40R18

Size	Wheel	Diameter	Difference
195/60R16	Minus 2 (16 in)	25.21 in	+0.50%
235/50R16	Minus 2 (16 in)	25.25 in	+0.66%
205/55R16	Minus 2 (16 in)	24.88 in	−0.83%
215/55R16	Minus 2 (16 in)	25.31 in	+0.89%
205/50R17	Minus 1 (17 in)	25.07 in	−0.06%
255/40R17	Minus 1 (17 in)	25.03 in	−0.22%
225/45R17	Minus 1 (17 in)	24.97 in	−0.46%
235/45R17	Minus 1 (17 in)	25.33 in	+0.96%
255/30R19	Plus 1 (19 in)	25.02 in	−0.25%
225/35R19	Plus 1 (19 in)	25.20 in	+0.46%
265/30R19	Plus 1 (19 in)	25.26 in	+0.69%
245/30R19	Plus 1 (19 in)	24.79 in	−1.19%
245/30R20	Plus 2 (20 in)	25.79 in	+2.79%

Vehicles approved with 225/40R18 tires

Models whose Swiss type approval lists 225/40R18 as a standard or optional size (12,003 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 225/40R18 size

225 — section width in millimetres (8.86 in).
40 — aspect ratio: the sidewall is 40 % of the width, 90.0 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.