255/40R21 Tire Size in Inches

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

255/40R21
29.0" tall × 10.0" wide
On a 21-inch wheel · roughly 29.0x10.0R21 in flotation sizing
MeasurementInchesMillimetres
Overall diameter29.03737
Section width10.04255
Sidewall height4.02102.0
Wheel diameter21533
Circumference91.22317
Revolutions per mile695 (432 per km)

What 255/40R21 means

255 is the section width in millimetres (10.0 in). 40 is the aspect ratio: the sidewall is 40% as tall as the tire is wide, which works out to 102.0 mm or 4.0 in. R21 is a radial tire for a 21-inch wheel.

Overall diameter is the wheel plus two sidewalls: 21 + 2 × 4.02 = 29.0 inches. These are the nominal dimensions every maker builds to; a real 255/40R21 can measure a few tenths of an inch differently by model and inflation.

Closest equivalent sizes

Common sizes within the usual 3% of 255/40R21's overall diameter. Confirm load index, speed rating and rim width before swapping.

SizeDiameterWidthChangeSpeedometer at 60
235/65R17 new wheels 29.0 in 9.3 in -0.0% really 60.0 mph
255/45R20 new wheels 29.0 in 10.0 in +0.0% really 60.0 mph
255/50R19 new wheels 29.0 in 10.0 in +0.0% really 60.0 mph
255/55R18 new wheels 29.0 in 10.0 in +0.0% really 60.0 mph
255/60R17 new wheels 29.0 in 10.0 in +0.1% really 60.0 mph
255/65R16 new wheels 29.1 in 10.0 in +0.1% really 60.0 mph

Speedometer if you switch to 235/65R17

Your speedometer stays calibrated for 255/40R21. 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 255/40R21 with any size →

Shop 255/40R21 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 255/40R21 tire is 29.0 inches (737 mm) tall overall. That is the 21-inch wheel plus two sidewalls of 4.0 inches each (40% 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, 255/40R21 is roughly a 29.0x10.0R21.

235/65R17 is the closest common size, at -0.0% overall diameter (29.0 in against 29.0 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 695 revolutions per mile (432 per kilometre), from its 29.0-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 265/40R21 265/45R21 275/40R21 275/45R21 285/40R21 295/35R21 275/35R21 315/35R21