GarageCalc What 235/55R20 means
235 is the section width in millimeters (9.3 in). 55 is the aspect ratio: the sidewall is 55 percent of the width, so 235 × 0.55 = 129 mm (5.1 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 × 129 = 767 mm, or 30.2 in.
A full marking adds a load index and speed rating after the size, for example 235/55R20 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 235/55R20
Going up one wheel size, the closest match is 255/45R21 at -0.5% diameter with a 115 mm sidewall instead of 129 mm. Going down one size, 255/55R19 is -0.4% with a taller 140 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 235/55R20 tire?
30.2 inches (767 mm) in overall diameter: the 20-inch wheel (508 mm) plus two 129 mm sidewalls. Real tires land within about 1 to 2 percent of that depending on brand, tread depth and pressure.
What is 235/55R20 in inches?
30.2 x 9.3 R20: 30.2 inches tall, 9.3 inches wide, on a 20-inch wheel. The sidewall is 5.1 inches and the circumference is 94.8 inches, so the tire turns 668 times per mile.
What tire sizes are equivalent to 235/55R20?
Within 3 percent of the same diameter on the same 20-inch wheel: 255/50R20 (-0.5%), 245/50R20 (-1.8%), 255/55R20 (+2.9%); on a 21-inch wheel: 255/45R21 (-0.5%), 245/45R21 (-1.6%); on a 19-inch wheel: 255/55R19 (-0.4%), 245/55R19 (-1.9%). The percentage is the diameter change, which is also your speedometer error.
Can I use 245/50R20 instead of 235/55R20?
Dimensionally yes: 245/50R20 is 1.8% shorter (-14 mm) and 10 mm wider, and at a true 60 mph the speedometer would read 61.1 mph. The extra width needs about 0.20 in of clearance on each side of the tire and a rim width that suits 245 mm.