GarageCalc What 305/60R20 means
305 is the section width in millimeters (12.0 in). 60 is the aspect ratio: the sidewall is 60 percent of the width, so 305 × 0.60 = 183 mm (7.2 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 × 183 = 874 mm, or 34.4 in.
A full marking adds a load index and speed rating after the size, for example 305/60R20 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 305/60R20
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 34.4 in, so 305/60R20 is best replaced like for like. The comparison calculator compares nominal dimensions for any candidate.
Frequently asked questions
How tall is a 305/60R20 tire?
34.4 inches (874 mm) in overall diameter: the 20-inch wheel (508 mm) plus two 183 mm sidewalls. Actual dimensions depend on the tire model, tread depth, pressure and load; check the tire manufacturer specifications.
What is 305/60R20 in inches?
34.4 x 12.0 R20: 34.4 inches tall, 12.0 inches wide, on a 20-inch wheel. The sidewall is 7.2 inches and the circumference is 108.1 inches, so the tire turns 586 times per mile.
What tire sizes are equivalent to 305/60R20?
Within 3 percent of the same diameter on the same 20-inch wheel: 285/65R20 (+0.5%), 295/60R20 (-1.4%), 295/65R20 (+2.0%). The percentage is the diameter change, which is also your speedometer error.
Can I use 325/60R20 instead of 305/60R20?
Nominal size comparison only: 325/60R20 is 2.7% taller (+24 mm) and 20 mm wider, and at a true 60 mph the speedometer would read 58.4 mph. This does not confirm fitment. Check approved rim widths, load and speed ratings, and vehicle clearances. The nominal extra width adds about 0.39 in of clearance on each side of the tire and a rim width that suits 325 mm.