GarageCalc What 305/55R20 means
305 is the section width in millimeters (12.0 in). 55 is the aspect ratio: the sidewall is 55 percent of the width, so 305 × 0.55 = 168 mm (6.6 in). R is radial construction, and 20 is the wheel diameter in inches (508 mm). Stack two sidewalls on the wheel and you get the overall diameter: 508 + 2 × 168 = 844 mm, or 33.2 in.
A full marking adds a load index and speed rating after the size, for example 305/55R20 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 305/55R20
Similar diameter does not establish interchangeability. Follow the vehicle manufacturer's approved sizes; see Michelin's tire-size guidance.
No common size on a 19 or 21-inch wheel stays within 3 percent of 33.2 in, so 305/55R20 is best replaced like for like. The comparison calculator compares nominal dimensions for any candidate.
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 305/55R20 tire?
33.2 inches (844 mm) in overall diameter: the 20-inch wheel (508 mm) plus two 168 mm sidewalls. Actual dimensions depend on the tire model, tread depth, pressure and load; check the tire manufacturer specifications.
What is 305/55R20 in inches?
33.2 x 12.0 R20: 33.2 inches tall, 12.0 inches wide, on a 20-inch wheel. The sidewall is 6.6 inches and the circumference is 104.3 inches, so the tire turns 607 times per mile.
What tire sizes are equivalent to 305/55R20?
Within 3 percent of the same diameter on the same 20-inch wheel: 285/60R20 (+0.8%), 295/55R20 (-1.3%), 295/60R20 (+2.2%). These are nominal diameter comparisons, not approved replacements. Check vehicle-approved sizes, rim width, load and speed ratings, and clearance before changing size.