GarageCalc What 255/55R20 means
255 is the section width in millimeters (10.0 in). 55 is the aspect ratio: the sidewall is 55 percent of the width, so 255 × 0.55 = 140 mm (5.5 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 × 140 = 789 mm, or 31.0 in.
A full marking adds a load index and speed rating after the size, for example 255/55R20 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 255/55R20
Going up one wheel size, the closest match is 255/50R21 at -0.0% diameter with a 128 mm sidewall instead of 140 mm. Going down one size, 275/55R19 is -0.4% with a taller 151 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 255/55R20 tire?
31.0 inches (789 mm) in overall diameter: the 20-inch wheel (508 mm) plus two 140 mm sidewalls. Real tires land within about 1 to 2 percent of that depending on brand, tread depth and pressure.
What is 255/55R20 in inches?
31.0 x 10.0 R20: 31.0 inches tall, 10.0 inches wide, on a 20-inch wheel. The sidewall is 5.5 inches and the circumference is 97.5 inches, so the tire turns 650 times per mile.
What tire sizes are equivalent to 255/55R20?
Within 3 percent of the same diameter on the same 20-inch wheel: 275/50R20 (-0.7%), 245/60R20 (+1.7%), 265/50R20 (-2.0%); on a 21-inch wheel: 255/50R21 (-0.0%), 275/45R21 (-1.0%); on a 19-inch wheel: 275/55R19 (-0.4%), 265/55R19 (-1.8%). The percentage is the diameter change, which is also your speedometer error.
Can I use 265/50R20 instead of 255/55R20?
Dimensionally yes: 265/50R20 is 2.0% shorter (-16 mm) and 10 mm wider, and at a true 60 mph the speedometer would read 61.2 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.