Metric tire size

255/40R18 tire size

A 255/40R18 tire is 26.0 inches (661 mm) tall and 10.0 in wide, for a 18-inch wheel, with a 4.0 in sidewall. That is the nominal size worked out from the code. Makers publish 25.8–26.2 in for real 255/40R18 tires (5 makers).

Diameter26.0 in661 mm

Width10.0 in255 mm

Sidewall4.0 in102 mm

Revs per mile775nominal, published 794–805

Wheel width8.5–10 inapproved by every listing

Compare 255/40R18 with other sizes

Set up for: comparing 255/40R18 with a size you're considering · Different situation?

Your current size

Current tire size

e.g. 265/70R17, 2657017 or 33x12.50R20

Where do I find my size?

It's on the tire sidewall, e.g. P265/70R17 115T, and on the tire placard sticker on the driver's door jamb (also in the owner's manual). The placard shows the size the vehicle came with; the sidewall shows what is fitted now. If they differ, use the placard as your starting point. Digits are enough here: 2657017 or 33125020.

Size you're considering

Size you're considering (2)

Real 255/40R18 tires: published diameters

Figures from the makers' own data books. They differ from the 26.0 in calculated figure because each tread design and construction measures slightly differently.

Maker (data book)	Published diameter	Revs per mile	Approved wheel width	Listings
Falken	26–26.2 in	794–800	8.5–10 in	5
Goodyear	26 in	803	8.5–10 in	1
Hankook	25.8–25.9 in	801–805	8.5–10 in	3
Nitto	25.98–26.06 in	—	8.5–10 in	2
Toyo	26 in	798	8.5–10 in	1

Sources: maker data books (PDF), checked 2026-11-05. Brand names identify the data source only.

Equivalent sizes on other wheels

Sizes that appear in the maker data books with a diameter within 3% of 255/40R18. Same diameter is only one part of fitting a different size.

Wheel	Real sizes within 3% (closest first)
15-inch	215/65R15 (26.0 in, −0.1%), 235/60R15 (26.1 in, +0.3%), 275/50R15 (25.8 in, −0.8%)
16-inch	255/50R16 (26.0 in, ±0.0%), 215/60R16 (26.2 in, +0.5%), 225/55R16 (25.7 in, −1.1%)
17-inch	255/45R17 (26.0 in, ±0.0%), 285/40R17 (26.0 in, −0.2%), 205/55R17 (25.9 in, −0.6%)
18-inch	225/45R18 (26.0 in, −0.2%), 295/35R18 (26.1 in, +0.4%), 335/30R18 (25.9 in, −0.5%)
20-inch	255/30R20 (26.0 in, ±0.0%), 305/25R20 (26.0 in, −0.1%), 225/35R20 (26.2 in, +0.7%)

Popular comparisons with 255/40R18

255/40R18 vs 265/35R18−2.8% diameter

FAQ

How tall is a 255/40R18 tire?

26.0 inches (661 mm) calculated from the size code. Makers publish 25.8–26.2 in for real tires in this size; the difference comes from tread design and construction.

How do you work out the 255/40R18 diameter?

Sidewall = 255 mm × 40% = 102.0 mm. Diameter = 18 in wheel + 2 × 102.0 mm ÷ 25.4 = 26.03 in.

Is 255/40R18 a 26-inch tire?

It is about 26.0 in tall nominally, so it is close to 26 inches. Real tires in a size vary slightly by model; check the maker's spec sheet for the exact tire.

What sizes are equivalent to 255/40R18?

Real sizes within 3% of its diameter include 255/50R16 (±0.0%), 255/45R17 (±0.0%) and 255/30R20 (±0.0%). Equal diameter alone doesn't make a size suitable: wheel width, offset, clearance and load rating all matter.

How many revolutions per mile does a 255/40R18 make?

About 775 calculated from its unloaded diameter. Makers publish 794–805 revs per mile, measured under load (a loaded tire rolls on a slightly smaller radius), which is why published figures are higher.

Open 255/40R18 in the converter · Tire size chart

Important: These are calculated (nominal) dimensions, not a fitment recommendation. Real tires of the same size differ by brand and model (makers allow about ±3% of section height), and width changes with the rim it is mounted on. The math can't see wheel width and offset, clearance at full steering lock and suspension travel, load index, speed rating, or what your vehicle maker approves. Before changing size, check your door-jamb placard and owner's manual and ask a tire professional. A replacement tire must meet or exceed the load index and speed rating on the placard.