285/40R21 Tire Size in Inches

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

285/40R21
30.0" tall × 11.2" wide
On a 21-inch wheel · roughly 30.0x11.2R21 in flotation sizing
MeasurementInchesMillimetres
Overall diameter29.98761
Section width11.22285
Sidewall height4.49114.0
Wheel diameter21533
Circumference94.22392
Revolutions per mile673 (418 per km)

What 285/40R21 means

285 is the section width in millimetres (11.2 in). 40 is the aspect ratio: the sidewall is 40% as tall as the tire is wide, which works out to 114.0 mm or 4.5 in. R21 is a radial tire for a 21-inch wheel.

Overall diameter is the wheel plus two sidewalls: 21 + 2 × 4.49 = 30.0 inches. These are the nominal dimensions every maker builds to; a real 285/40R21 can measure a few tenths of an inch differently by model and inflation.

Closest equivalent sizes

Common sizes within the usual 3% of 285/40R21's overall diameter. Confirm load index, speed rating and rim width before swapping.

SizeDiameterWidthChangeSpeedometer at 60
275/60R17 new wheels 30.0 in 10.8 in +0.1% really 60.0 mph
235/70R17 new wheels 30.0 in 9.3 in -0.1% really 60.0 mph
235/65R18 new wheels 30.0 in 9.3 in +0.2% really 60.1 mph
255/50R20 new wheels 30.0 in 10.0 in +0.2% really 60.1 mph
255/55R19 new wheels 30.0 in 10.0 in +0.2% really 60.1 mph
255/60R18 new wheels 30.0 in 10.0 in +0.2% really 60.1 mph

Speedometer if you switch to 275/60R17

Your speedometer stays calibrated for 285/40R21. With a taller tire you travel faster 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 285/40R21 with any size →

Shop 285/40R21 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 285/40R21 tire is 30.0 inches (761 mm) tall overall. That is the 21-inch wheel plus two sidewalls of 4.5 inches each (40% of the 285 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 285 mm, or 11.2 inches, measured sidewall to sidewall on the recommended rim width. The tread itself is narrower than that. In flotation sizing, 285/40R21 is roughly a 30.0x11.2R21.

275/60R17 is the closest common size, at +0.1% overall diameter (30.0 in against 30.0 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 673 revolutions per mile (418 per kilometre), from its 30.0-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 21-inch sizes

245/45R21 255/40R21 265/40R21 265/45R21 275/40R21 275/45R21 295/35R21 275/35R21 315/35R21