GarageCalc What 245/35R20 means
245 is the section width in millimeters (9.6 in). 35 is the aspect ratio: the sidewall is 35 percent of the width, so 245 × 0.35 = 86 mm (3.4 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 × 86 = 680 mm, or 26.8 in.
A full marking adds a load index and speed rating after the size, for example 245/35R20 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 245/35R20
No common 21-inch size stays within 3 percent of 26.8 in. Going down one size, 245/40R19 is -0.1% with a taller 98 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 245/35R20 tire?
26.8 inches (680 mm) in overall diameter: the 20-inch wheel (508 mm) plus two 86 mm sidewalls. Real tires land within about 1 to 2 percent of that depending on brand, tread depth and pressure.
What is 245/35R20 in inches?
26.8 x 9.6 R20: 26.8 inches tall, 9.6 inches wide, on a 20-inch wheel. The sidewall is 3.4 inches and the circumference is 84.0 inches, so the tire turns 754 times per mile.
What tire sizes are equivalent to 245/35R20?
Within 3 percent of the same diameter on the same 20-inch wheel: 235/35R20 (-1.0%), 255/35R20 (+1.0%), 225/35R20 (-2.1%); on a 19-inch wheel: 245/40R19 (-0.1%), 255/40R19 (+1.0%). The percentage is the diameter change, which is also your speedometer error.
Can I use 255/35R20 instead of 245/35R20?
Dimensionally yes: 255/35R20 is 1.0% taller (+7 mm) and 10 mm wider, and at a true 60 mph the speedometer would read 59.4 mph. The extra width needs about 0.20 in of clearance on each side of the tire and a rim width that suits 255 mm.