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
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 checks any candidate against the 3 percent guideline.
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. Real tires land within about 1 to 2 percent of that depending on brand, tread depth and pressure.
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?
Dimensionally yes: 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. The extra width needs about 0.39 in of clearance on each side of the tire and a rim width that suits 325 mm.