GarageCalc What 225/35R20 means
225 is the section width in millimeters (8.9 in). 35 is the aspect ratio: the sidewall is 35 percent of the width, so 225 × 0.35 = 79 mm (3.1 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 × 79 = 666 mm, or 26.2 in.
A full marking adds a load index and speed rating after the size, for example 225/35R20 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 225/35R20
No common 21-inch size stays within 3 percent of 26.2 in. Going down one size, 225/40R19 is -0.4% with a taller 90 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 225/35R20 tire?
26.2 inches (666 mm) in overall diameter: the 20-inch wheel (508 mm) plus two 79 mm sidewalls. Real tires land within about 1 to 2 percent of that depending on brand, tread depth and pressure.
What is 225/35R20 in inches?
26.2 x 8.9 R20: 26.2 inches tall, 8.9 inches wide, on a 20-inch wheel. The sidewall is 3.1 inches and the circumference is 82.3 inches, so the tire turns 770 times per mile.
What tire sizes are equivalent to 225/35R20?
Within 3 percent of the same diameter on the same 20-inch wheel: 235/35R20 (+1.1%), 245/30R20 (-1.6%), 245/35R20 (+2.1%); on a 19-inch wheel: 225/40R19 (-0.4%), 235/40R19 (+0.8%). The percentage is the diameter change, which is also your speedometer error.
Can I use 235/35R20 instead of 225/35R20?
Dimensionally yes: 235/35R20 is 1.1% 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 235 mm.