GarageCalc What 305/40R22 means
305 is the section width in millimeters (12.0 in). 40 is the aspect ratio: the sidewall is 40 percent of the width, so 305 × 0.40 = 122 mm (4.8 in). R is radial construction, and 22 is the wheel diameter in inches (559 mm). Stack two sidewalls on the wheel and you get the overall diameter: 559 + 2 × 122 = 803 mm, or 31.6 in.
A full marking adds a load index and speed rating after the size, for example 305/40R22 91V. Those two codes set how much weight and speed the tire is certified for; look them up in the load index chart and the speed rating chart.
Plus and minus sizing from 305/40R22
Similar diameter does not establish interchangeability. Follow the vehicle manufacturer's approved sizes; see Michelin's tire-size guidance.
No common 23-inch size stays within 3 percent of 31.6 in. Going down one size, 285/45R21 is -1.6% with a taller 128 mm sidewall for more comfort and pothole margin. The 3 percent cutoff is a comparison filter, not a fitment approval. Use vehicle-approved sizes and check rim width, load and speed ratings, and clearances. The comparison calculator shows the full speedometer table for any pair.
For the effect on road speed and distance, see our worked speedometer example. When maintaining this set, use the rotation pattern guide to check whether tire direction or different front and rear sizes restrict rotation.
Frequently asked questions
How tall is a 305/40R22 tire?
31.6 inches (803 mm) in overall diameter: the 22-inch wheel (559 mm) plus two 122 mm sidewalls. Actual dimensions depend on the tire model, tread depth, pressure and load; check the tire manufacturer specifications.
What is 305/40R22 in inches?
31.6 x 12.0 R22: 31.6 inches tall, 12.0 inches wide, on a 22-inch wheel. The sidewall is 4.8 inches and the circumference is 99.3 inches, so the tire turns 638 times per mile.
What tire sizes are equivalent to 305/40R22?
Within 3 percent of the same diameter on the same 22-inch wheel: 285/45R22 (+1.6%), 285/40R22 (-2.0%); on a 21-inch wheel: 285/45R21 (-1.6%). These are nominal diameter comparisons, not approved replacements. Check vehicle-approved sizes, rim width, load and speed ratings, and clearance before changing size.