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
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 checks any candidate against the 3 percent guideline.
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. Real tires land within about 1 to 2 percent of that depending on brand, tread depth and pressure.
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%). The percentage is the diameter change, which is also your speedometer error.