255/40R18 tire size

A 255/40R18 tire is 26.03 inches (661.2 mm) tall, 10.04 in (255 mm) wide and fits a 18-inch wheel. In inch notation it is roughly a 26.0x10.0R18.

255/40R18 dimensions
Overall diameter	26.03 in	661.2 mm
Section width	10.04 in	255 mm
Sidewall height	4.02 in	102.0 mm
Circumference	81.78 in	2,077 mm
Revolutions	774.8 per mile	481.4 per km
Rim diameter	18 in	457.2 mm

Compare 255/40R18 with another size

Current size

New size

Speed in mphkm/h

Overall diameter difference

−0.23%

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: 775 → 777 per mile

255/40R18 225/45R18

Side-by-side dimensions

	Current	New	Difference	%
Diameter	26.03 in	25.97 in	−0.06 in	−0.23%
Diameter	661.2 mm	659.7 mm	−1.5 mm	−0.23%
Width	10.04 in	8.86 in	−1.18 in	−11.76%
Width	255.0 mm	225.0 mm	−30.0 mm	−11.76%
Sidewall	4.02 in	3.99 in	−0.03 in	−0.74%
Sidewall	102.0 mm	101.3 mm	−0.8 mm	−0.74%
Circumference	81.78 in	81.59 in	−0.19 in	−0.23%
Revs per mile	774.8	776.5	+1.8	+0.23%

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 255/40R18:

Plus and minus sizes for 255/40R18

Size	Wheel	Diameter	Difference
225/55R16	Minus 2 (16 in)	25.74 in	−1.10%
245/50R16	Minus 2 (16 in)	25.65 in	−1.48%
245/55R16	Minus 2 (16 in)	26.61 in	+2.22%
225/60R16	Minus 2 (16 in)	26.63 in	+2.30%
255/45R17	Minus 1 (17 in)	26.04 in	+0.02%
285/40R17	Minus 1 (17 in)	25.98 in	−0.21%
225/50R17	Minus 1 (17 in)	25.86 in	−0.67%
235/50R17	Minus 1 (17 in)	26.25 in	+0.85%
255/35R19	Plus 1 (19 in)	26.03 in	−0.02%
225/40R19	Plus 1 (19 in)	26.09 in	+0.21%
295/30R19	Plus 1 (19 in)	25.97 in	−0.24%
305/30R19	Plus 1 (19 in)	26.20 in	+0.67%
255/30R20	Plus 2 (20 in)	26.02 in	−0.03%
225/35R20	Plus 2 (20 in)	26.20 in	+0.65%
265/30R20	Plus 2 (20 in)	26.26 in	+0.88%
245/30R20	Plus 2 (20 in)	25.79 in	−0.94%
295/25R21	Plus 3 (21 in)	26.81 in	+2.98%

Vehicles approved with 255/40R18 tires

Models whose Swiss type approval lists 255/40R18 as a standard or optional size (5,434 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 255/40R18 size

255 — section width in millimetres (10.04 in).
40 — aspect ratio: the sidewall is 40 % of the width, 102.0 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.