GarageCalc What 315/35R20 means
315 is the section width in millimeters (12.4 in). 35 is the aspect ratio: the sidewall is 35 percent of the width, so 315 × 0.35 = 110 mm (4.3 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 × 110 = 729 mm, or 28.7 in.
A full marking adds a load index and speed rating after the size, for example 315/35R20 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 315/35R20
Going up one wheel size, the closest match is 295/35R21 at +1.6% diameter with a 103 mm sidewall instead of 110 mm. Going down one size, 295/40R19 is -1.4% with a taller 118 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 315/35R20 tire?
28.7 inches (729 mm) in overall diameter: the 20-inch wheel (508 mm) plus two 110 mm sidewalls. Real tires land within about 1 to 2 percent of that depending on brand, tread depth and pressure.
What is 315/35R20 in inches?
28.7 x 12.4 R20: 28.7 inches tall, 12.4 inches wide, on a 20-inch wheel. The sidewall is 4.3 inches and the circumference is 90.1 inches, so the tire turns 703 times per mile.
What tire sizes are equivalent to 315/35R20?
Within 3 percent of the same diameter on the same 20-inch wheel: 305/35R20 (-1.0%), 295/35R20 (-1.9%), 295/40R20 (+2.1%); on a 21-inch wheel: 295/35R21 (+1.6%), 305/35R21 (+2.5%); on a 19-inch wheel: 295/40R19 (-1.4%). The percentage is the diameter change, which is also your speedometer error.