GarageCalc What 255/50R20 means
255 is the section width in millimeters (10.0 in). 50 is the aspect ratio: the sidewall is 50 percent of the width, so 255 × 0.50 = 128 mm (5.0 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 × 128 = 763 mm, or 30.0 in.
A full marking adds a load index and speed rating after the size, for example 255/50R20 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 255/50R20
Going up one wheel size, the closest match is 255/45R21 at -0.0% diameter with a 115 mm sidewall instead of 128 mm. Going down one size, 255/55R19 is +0.0% 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 255/50R20 tire?
30.0 inches (763 mm) in overall diameter: the 20-inch wheel (508 mm) plus two 128 mm sidewalls. Real tires land within about 1 to 2 percent of that depending on brand, tread depth and pressure.
What is 255/50R20 in inches?
30.0 x 10.0 R20: 30.0 inches tall, 10.0 inches wide, on a 20-inch wheel. The sidewall is 5.0 inches and the circumference is 94.4 inches, so the tire turns 671 times per mile.
What tire sizes are equivalent to 255/50R20?
Within 3 percent of the same diameter on the same 20-inch wheel: 235/55R20 (+0.5%), 275/45R20 (-1.0%), 245/50R20 (-1.3%); on a 21-inch wheel: 255/45R21 (-0.0%), 265/45R21 (+1.2%); on a 19-inch wheel: 255/55R19 (+0.0%), 275/50R19 (-0.7%). The percentage is the diameter change, which is also your speedometer error.
Can I use 265/50R20 instead of 255/50R20?
Dimensionally yes: 265/50R20 is 1.3% taller (+10 mm) and 10 mm wider, and at a true 60 mph the speedometer would read 59.2 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.