GarageCalc What 305/30R20 means
305 is the section width in millimeters (12.0 in). 30 is the aspect ratio: the sidewall is 30 percent of the width, so 305 × 0.30 = 92 mm (3.6 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 × 92 = 691 mm, or 27.2 in.
A full marking adds a load index and speed rating after the size, for example 305/30R20 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 305/30R20
No common 21-inch size stays within 3 percent of 27.2 in. Going down one size, 295/35R19 is -0.3% with a taller 103 mm sidewall for more comfort and pothole margin. Anything inside 3 percent keeps the speedometer, ABS and gearing effectively stock; the comparison calculator shows the full speedometer table for any pair.
Frequently asked questions
How tall is a 305/30R20 tire?
27.2 inches (691 mm) in overall diameter: the 20-inch wheel (508 mm) plus two 92 mm sidewalls. Real tires land within about 1 to 2 percent of that depending on brand, tread depth and pressure.
What is 305/30R20 in inches?
27.2 x 12.0 R20: 27.2 inches tall, 12.0 inches wide, on a 20-inch wheel. The sidewall is 3.6 inches and the circumference is 85.5 inches, so the tire turns 741 times per mile.
What tire sizes are equivalent to 305/30R20?
Within 3 percent of the same diameter on the same 20-inch wheel: 295/30R20 (-0.9%), 315/30R20 (+0.9%), 285/30R20 (-1.7%); on a 19-inch wheel: 295/35R19 (-0.3%), 305/35R19 (+0.7%). The percentage is the diameter change, which is also your speedometer error.
Can I use 315/30R20 instead of 305/30R20?
Dimensionally yes: 315/30R20 is 0.9% taller (+6 mm) and 10 mm wider, and at a true 60 mph the speedometer would read 59.5 mph. The extra width needs about 0.20 in of clearance on each side of the tire and a rim width that suits 315 mm.