225/40R18 tire size in inches

Nominal 225/40R18 dimensions

Overall diameter

25.09 in637.2 mm

Section width

8.86 in225 mm

Sidewall height

3.54 in90.0 mm

Circumference

78.81 in2,001.8 mm

Rim diameter

18 in

Revolutions per mile

804nominal, from geometric circumference

Inch notation

25.1x8.9R18approximate

Where 225/40R18 fits

• Inch class: 25.09 in is within 0.1 in of a true 25 inches, so the 25-inch label is close to exact.
• Profile: the 18-inch rim plus two 3.54 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 18-inch rim: 225/40R18 is the shortest 18-inch size in the catalog. The next taller is 245/40R18 (25.72 in, +2.51%).
• Same 225 mm width: the closest in diameter are 225/50R16 (24.86 in, -0.91%), 225/45R17 (24.97 in, -0.46%), 225/55R16 (25.74 in, +2.62%), 225/50R17 (25.86 in, +3.08%). At the same width, a different profile or rim changes sidewall height and overall diameter, not section width.

Reading the marking: 225 is the section width in millimeters, 40 is the sidewall height as a percentage of that width, and R18 means radial construction for a 18-inch rim. Percentages are measured from 225/40R18.

Similar sizes on other rim diameters

Within 3% of 225/40R18's overall diameter, nearest first. The speed column is the actual speed when a speedometer calibrated for 225/40R18 reads 60 mph.

For a size without a comparison page, open 225/40R18 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 225/40R18

None keeps the 18-inch rim; each one changes wheel diameter. Diameter changes are measured from 225/40R18.