285/40R18 tire size

A 285/40R18 tire is 26.98 inches (685.2 mm) tall, 11.22 in (285 mm) wide and fits a 18-inch wheel. In inch notation it is roughly a 27.0x11.2R18.

285/40R18 dimensions
Overall diameter	26.98 in	685.2 mm
Section width	11.22 in	285 mm
Sidewall height	4.49 in	114.0 mm
Circumference	84.75 in	2,153 mm
Revolutions	747.6 per mile	464.6 per km
Rim diameter	18 in	457.2 mm

Compare 285/40R18 with another size

Current size

New size

Speed in mphkm/h

Overall diameter difference

+0.22%

60 mph on the speedometer = 60.1 mph actual

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

Ground clearance: +0.8 mm (+0.03 in)
Revolutions: 748 → 746 per mile

285/40R18 255/45R18

Side-by-side dimensions

	Current	New	Difference	%
Diameter	26.98 in	27.04 in	+0.06 in	+0.22%
Diameter	685.2 mm	686.7 mm	+1.5 mm	+0.22%
Width	11.22 in	10.04 in	−1.18 in	−10.53%
Width	285.0 mm	255.0 mm	−30.0 mm	−10.53%
Sidewall	4.49 in	4.52 in	+0.03 in	+0.66%
Sidewall	114.0 mm	114.8 mm	+0.8 mm	+0.66%
Circumference	84.75 in	84.93 in	+0.19 in	+0.22%
Revs per mile	747.6	746.0	−1.6	−0.22%

Speedometer error

Speedometer shows	Actual speed	Difference
20 mph	20.0 mph	0.0 mph
30 mph	30.1 mph	+0.1 mph
40 mph	40.1 mph	+0.1 mph
50 mph	50.1 mph	+0.1 mph
60 mph	60.1 mph	+0.1 mph
70 mph	70.2 mph	+0.2 mph
80 mph	80.2 mph	+0.2 mph

Alternative sizes on the same 18-inch wheel

Sizes in use within ±3 % of the overall diameter of 285/40R18:

Plus and minus sizes for 285/40R18

Size	Wheel	Diameter	Difference
335/35R17	Minus 1 (17 in)	26.23 in	−2.76%
255/40R19	Plus 1 (19 in)	27.03 in	+0.20%
285/35R19	Plus 1 (19 in)	26.85 in	−0.45%
295/35R19	Plus 1 (19 in)	27.13 in	+0.57%
325/30R19	Plus 1 (19 in)	26.68 in	−1.11%
295/30R20	Plus 2 (20 in)	26.97 in	−0.03%
255/35R20	Plus 2 (20 in)	27.03 in	+0.19%
305/30R20	Plus 2 (20 in)	27.20 in	+0.85%
285/30R20	Plus 2 (20 in)	26.73 in	−0.90%
295/25R21	Plus 3 (21 in)	26.81 in	−0.63%
265/30R21	Plus 3 (21 in)	27.26 in	+1.05%
275/30R21	Plus 3 (21 in)	27.50 in	+1.93%
285/30R21	Plus 3 (21 in)	27.73 in	+2.80%

Vehicles approved with 285/40R18 tires

Models whose Swiss type approval lists 285/40R18 as a standard or optional size (5 approvals in total). Always check your own vehicle's placard or papers.

Maserati Quattroporte

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 285/40R18 size

285 — section width in millimetres (11.22 in).
40 — aspect ratio: the sidewall is 40 % of the width, 114.0 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.