GarageCalc What 305/50R20 means
305 is the section width in millimeters (12.0 in). 50 is the aspect ratio: the sidewall is 50 percent of the width, so 305 × 0.50 = 153 mm (6.0 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 × 153 = 813 mm, or 32.0 in.
A full marking adds a load index and speed rating after the size, for example 305/50R20 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 305/50R20
Similar diameter does not establish interchangeability. Follow the vehicle manufacturer's approved sizes; see Michelin's tire-size guidance.
Going up one wheel size, the closest match is 285/45R21 at -2.8% diameter with a 128 mm sidewall instead of 153 mm. No common 19-inch size stays within 3 percent either. 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.
Frequently asked questions
How tall is a 305/50R20 tire?
32.0 inches (813 mm) in overall diameter: the 20-inch wheel (508 mm) plus two 153 mm sidewalls. Actual dimensions depend on the tire model, tread depth, pressure and load; check the tire manufacturer specifications.
What is 305/50R20 in inches?
32.0 x 12.0 R20: 32.0 inches tall, 12.0 inches wide, on a 20-inch wheel. The sidewall is 6.0 inches and the circumference is 100.6 inches, so the tire turns 630 times per mile.
What tire sizes are equivalent to 305/50R20?
Within 3 percent of the same diameter on the same 20-inch wheel: 285/55R20 (+1.0%), 295/55R20 (+2.4%), 285/50R20 (-2.5%); on a 21-inch wheel: 285/45R21 (-2.8%). The percentage is the diameter change, which is also your speedometer error.