225/40R18 Tire Size in Inches

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

225/40R18
25.1" tall × 8.9" wide
On a 18-inch wheel · roughly 25.1x8.9R18 in flotation sizing
MeasurementInchesMillimetres
Overall diameter25.09637
Section width8.86225
Sidewall height3.5490.0
Wheel diameter18457
Circumference78.82002
Revolutions per mile804 (500 per km)

What 225/40R18 means

225 is the section width in millimetres (8.9 in). 40 is the aspect ratio: the sidewall is 40% as tall as the tire is wide, which works out to 90.0 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 × 3.54 = 25.1 inches. These are the nominal dimensions every maker builds to; a real 225/40R18 can measure a few tenths of an inch differently by model and inflation.

Closest equivalent sizes

Common sizes within the usual 3% of 225/40R18's overall diameter. Confirm load index, speed rating and rim width before swapping.

SizeDiameterWidthChangeSpeedometer at 60
205/50R17 new wheels 25.1 in 8.1 in -0.1% really 60.0 mph
255/40R17 new wheels 25.0 in 10.0 in -0.2% really 59.9 mph
255/35R18 25.0 in 10.0 in -0.2% really 59.9 mph
215/60R15 new wheels 25.2 in 8.5 in +0.3% really 60.2 mph
195/65R15 new wheels 25.0 in 7.7 in -0.4% really 59.7 mph
225/45R17 new wheels 25.0 in 8.9 in -0.5% really 59.7 mph

Speedometer if you switch to 255/35R18

Your speedometer stays calibrated for 225/40R18. With a shorter tire you travel slower than it shows.

Speedometer readsTrue speed
30 mph29.9 mph
45 mph44.9 mph
55 mph54.9 mph
65 mph64.8 mph
75 mph74.8 mph
Compare 225/40R18 with any size →

Shop 225/40R18 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 225/40R18 tire is 25.1 inches (637 mm) tall overall. That is the 18-inch wheel plus two sidewalls of 3.5 inches each (40% of the 225 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 225 mm, or 8.9 inches, measured sidewall to sidewall on the recommended rim width. The tread itself is narrower than that. In flotation sizing, 225/40R18 is roughly a 25.1x8.9R18.

205/50R17 is the closest common size, at -0.1% overall diameter (25.1 in against 25.1 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 804 revolutions per mile (500 per kilometre), from its 25.1-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 225/45R18 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 255/45R18 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