GarageCalc What 315/30R20 means
315 is the section width in millimeters (12.4 in). 30 is the aspect ratio: the sidewall is 30 percent of the width, so 315 × 0.30 = 95 mm (3.7 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 × 95 = 697 mm, or 27.4 in.
A full marking adds a load index and speed rating after the size, for example 315/30R20 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 315/30R20
No common 21-inch size stays within 3 percent of 27.4 in. Going down one size, 305/35R19 is -0.1% with a taller 107 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 315/30R20 tire?
27.4 inches (697 mm) in overall diameter: the 20-inch wheel (508 mm) plus two 95 mm sidewalls. Real tires land within about 1 to 2 percent of that depending on brand, tread depth and pressure.
What is 315/30R20 in inches?
27.4 x 12.4 R20: 27.4 inches tall, 12.4 inches wide, on a 20-inch wheel. The sidewall is 3.7 inches and the circumference is 86.2 inches, so the tire turns 735 times per mile.
What tire sizes are equivalent to 315/30R20?
Within 3 percent of the same diameter on the same 20-inch wheel: 305/30R20 (-0.9%), 295/30R20 (-1.7%), 295/35R20 (+2.5%); on a 19-inch wheel: 305/35R19 (-0.1%), 295/35R19 (-1.1%). The percentage is the diameter change, which is also your speedometer error.