255/40R21 tire size in inches

Nominal 255/40R21 dimensions

Overall diameter

29.03 in737.4 mm

Section width

10.04 in255 mm

Sidewall height

4.02 in102.0 mm

Circumference

91.21 in2,316.6 mm

Rim diameter

21 in

Revolutions per mile

695nominal, from geometric circumference

Inch notation

29.0x10.0R21approximate

Where 255/40R21 fits

• Inch class: 29.03 in is within 0.1 in of a true 29 inches, so the 29-inch label is close to exact.
• Profile: the 21-inch rim plus two 4.02 in sidewalls make up the overall diameter, and the sidewalls are 28% of it. At 40 series that is a low profile: less sidewall between the rim and the road, so the ride is usually firmer and the wheel is more exposed to potholes and curbs.
• Same 21-inch rim: 255/40R21 is the tallest 21-inch size in the catalog. The next shorter is 285/35R21 (28.85 in, -0.61%).
• Same 255 mm width: the closest in diameter are 255/35R22 (29.03 in, -0.01%), 255/45R20 (29.04 in, +0.01%), 255/65R16 (29.05 in, +0.07%), 255/70R15 (29.06 in, +0.08%). At the same width, a different profile or rim changes sidewall height and overall diameter, not section width.

Reading the marking: 255 is the section width in millimeters, 40 is the sidewall height as a percentage of that width, and R21 means radial construction for a 21-inch rim. Percentages are measured from 255/40R21.

Similar sizes on other rim diameters

Within 3% of 255/40R21's overall diameter, nearest first. The speed column is the actual speed when a speedometer calibrated for 255/40R21 reads 60 mph.

For a size without a comparison page, open 255/40R21 in the calculator and enter that size as the proposed tire.

Important: These results compare nominal dimensions. They do not confirm vehicle fitment. Actual clearance varies by tire model, approved rim width, wheel construction, suspension, alignment, body tolerances, steering angle, load, and brake geometry. Verify load and speed ratings and physically test-fit the complete setup before driving.

Physical checks before changing size

• Approved rim-width range for the exact tire model
• Load index and speed rating equal to or greater than vehicle requirements
• Brake-caliper, hub, and wheel-fastener compatibility
• Fender, liner, suspension, and full-lock steering clearance
• Clearance at full compression and under expected load
• Alignment, tire pressure, and manufacturer guidance
• Professional inspection when variance or clearance is uncertain

Published comparisons for 255/40R21

3 of 5 keep the 21-inch rim. Diameter changes are measured from 255/40R21.