255/35R18 Tire Size in Inches

Diameter, width, sidewall and the closest equivalent sizes, computed from the size code.

255/35R18
25.0" tall × 10.0" wide
On a 18-inch wheel · roughly 25.0x10.0R18 in flotation sizing
MeasurementInchesMillimetres
Overall diameter25.03636
Section width10.04255
Sidewall height3.5189.2
Wheel diameter18457
Circumference78.61997
Revolutions per mile806 (501 per km)

What 255/35R18 means

255 is the section width in millimetres (10.0 in). 35 is the aspect ratio: the sidewall is 35% as tall as the tire is wide, which works out to 89.2 mm or 3.5 in. R18 is a radial tire for a 18-inch wheel.

Overall diameter is the wheel plus two sidewalls: 18 + 2 × 3.51 = 25.0 inches. These are the nominal dimensions every maker builds to; a real 255/35R18 can measure a few tenths of an inch differently by model and inflation.

Closest equivalent sizes

Common sizes within the usual 3% of 255/35R18's overall diameter. Confirm load index, speed rating and rim width before swapping.

SizeDiameterWidthChangeSpeedometer at 60
255/40R17 new wheels 25.0 in 10.0 in +0.0% really 60.0 mph
205/50R17 new wheels 25.1 in 8.1 in +0.2% really 60.1 mph
195/65R15 new wheels 25.0 in 7.7 in -0.2% really 59.9 mph
225/45R17 new wheels 25.0 in 8.9 in -0.2% really 59.9 mph
225/40R18 25.1 in 8.9 in +0.2% really 60.1 mph
215/60R15 new wheels 25.2 in 8.5 in +0.5% really 60.3 mph

Speedometer if you switch to 225/40R18

Your speedometer stays calibrated for 255/35R18. With a taller tire you travel faster than it shows.

Speedometer readsTrue speed
30 mph30.1 mph
45 mph45.1 mph
55 mph55.1 mph
65 mph65.2 mph
75 mph75.2 mph
Compare 255/35R18 with any size →

Shop 255/35R18 tires

Check the load index and speed rating on your door-jamb placard before you buy, especially for LT tires used for towing.

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Frequently Asked Questions

A 255/35R18 tire is 25.0 inches (636 mm) tall overall. That is the 18-inch wheel plus two sidewalls of 3.5 inches each (35% of the 255 mm section width). Expect a real tire to measure within a few tenths of an inch of this, depending on the model and inflation.

The section width is 255 mm, or 10.0 inches, measured sidewall to sidewall on the recommended rim width. The tread itself is narrower than that. In flotation sizing, 255/35R18 is roughly a 25.0x10.0R18.

255/40R17 is the closest common size, at +0.0% overall diameter (25.0 in against 25.0 in). Most makers treat about 3% as the limit for a swap without speedometer or clearance problems, but always confirm load index, speed rating and wheel width before changing size.

About 806 revolutions per mile (501 per kilometre), from its 25.0-inch rolling diameter. Real-world figures run slightly higher because a loaded tire squats. This number is what your speedometer and odometer are calibrated against.

Last updated: October 2, 2026

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Other 18-inch sizes

215/45R18 215/50R18 215/55R18 225/40R18 225/45R18 225/50R18 225/55R18 225/60R18 235/40R18 235/45R18 235/50R18 235/55R18 235/60R18 235/65R18 245/40R18 245/45R18 245/50R18 245/60R18 245/65R18 255/45R18 255/55R18 255/60R18 255/65R18 255/70R18 265/35R18 265/60R18 265/65R18 265/70R18 275/65R18 275/70R18 285/65R18 285/75R18 275/40R18 275/60R18 285/60R18 295/70R18