GarageCalc What 255/35R20 means
255 is the section width in millimeters (10.0 in). 35 is the aspect ratio: the sidewall is 35 percent of the width, so 255 × 0.35 = 89 mm (3.5 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 × 89 = 687 mm, or 27.0 in.
A full marking adds a load index and speed rating after the size, for example 255/35R20 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 255/35R20
No common 21-inch size stays within 3 percent of 27.0 in. Going down one size, 255/40R19 is +0.0% with a taller 102 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 255/35R20 tire?
27.0 inches (687 mm) in overall diameter: the 20-inch wheel (508 mm) plus two 89 mm sidewalls. Real tires land within about 1 to 2 percent of that depending on brand, tread depth and pressure.
What is 255/35R20 in inches?
27.0 x 10.0 R20: 27.0 inches tall, 10.0 inches wide, on a 20-inch wheel. The sidewall is 3.5 inches and the circumference is 84.9 inches, so the tire turns 746 times per mile.
What tire sizes are equivalent to 255/35R20?
Within 3 percent of the same diameter on the same 20-inch wheel: 245/35R20 (-1.0%), 265/35R20 (+1.0%), 275/30R20 (-2.0%); on a 19-inch wheel: 255/40R19 (+0.0%), 235/45R19 (+1.1%). The percentage is the diameter change, which is also your speedometer error.
Can I use 265/35R20 instead of 255/35R20?
Dimensionally yes: 265/35R20 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 265 mm.