255/45R18 Tire Size in Inches

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

255/45R18
27.0" tall × 10.0" wide
On a 18-inch wheel · roughly 27.0x10.0R18 in flotation sizing
MeasurementInchesMillimetres
Overall diameter27.04687
Section width10.04255
Sidewall height4.52114.8
Wheel diameter18457
Circumference84.92157
Revolutions per mile746 (464 per km)

What 255/45R18 means

255 is the section width in millimetres (10.0 in). 45 is the aspect ratio: the sidewall is 45% as tall as the tire is wide, which works out to 114.8 mm or 4.5 in. R18 is a radial tire for a 18-inch wheel.

Overall diameter is the wheel plus two sidewalls: 18 + 2 × 4.52 = 27.0 inches. These are the nominal dimensions every maker builds to; a real 255/45R18 can measure a few tenths of an inch differently by model and inflation.

Closest equivalent sizes

Common sizes within the usual 3% of 255/45R18's overall diameter. Confirm load index, speed rating and rim width before swapping.

SizeDiameterWidthChangeSpeedometer at 60
255/40R19 new wheels 27.0 in 10.0 in -0.0% really 60.0 mph
255/35R20 new wheels 27.0 in 10.0 in -0.0% really 60.0 mph
215/65R16 new wheels 27.0 in 8.5 in -0.1% really 59.9 mph
225/45R19 new wheels 27.0 in 8.9 in -0.2% really 59.9 mph
235/60R16 new wheels 27.1 in 9.3 in +0.2% really 60.1 mph
205/75R15 new wheels 27.1 in 8.1 in +0.3% really 60.2 mph

Speedometer if you switch to 255/40R19

Your speedometer stays calibrated for 255/45R18. With a shorter tire you travel slower than it shows.

Speedometer readsTrue speed
30 mph30.0 mph
45 mph45.0 mph
55 mph55.0 mph
65 mph65.0 mph
75 mph75.0 mph
Compare 255/45R18 with any size →

Shop 255/45R18 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 255/45R18 tire is 27.0 inches (687 mm) tall overall. That is the 18-inch wheel plus two sidewalls of 4.5 inches each (45% 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, 255/45R18 is roughly a 27.0x10.0R18.

255/40R19 is the closest common size, at -0.0% overall diameter (27.0 in against 27.0 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 746 revolutions per mile (464 per kilometre), from its 27.0-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 255/35R18 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