GarageCalc What 285/50R20 means
285 is the section width in millimeters (11.2 in). 50 is the aspect ratio: the sidewall is 50 percent of the width, so 285 × 0.50 = 143 mm (5.6 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 × 143 = 793 mm, or 31.2 in.
A full marking adds a load index and speed rating after the size, for example 285/50R20 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 285/50R20
Going up one wheel size, the closest match is 285/45R21 at -0.4% diameter with a 128 mm sidewall instead of 143 mm. Going down one size, 275/55R19 is -1.0% 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 285/50R20 tire?
31.2 inches (793 mm) in overall diameter: the 20-inch wheel (508 mm) plus two 143 mm sidewalls. Real tires land within about 1 to 2 percent of that depending on brand, tread depth and pressure.
What is 285/50R20 in inches?
31.2 x 11.2 R20: 31.2 inches tall, 11.2 inches wide, on a 20-inch wheel. The sidewall is 5.6 inches and the circumference is 98.1 inches, so the tire turns 646 times per mile.
What tire sizes are equivalent to 285/50R20?
Within 3 percent of the same diameter on the same 20-inch wheel: 275/50R20 (-1.3%), 275/55R20 (+2.2%), 295/45R20 (-2.5%); on a 21-inch wheel: 285/45R21 (-0.4%), 275/45R21 (-1.5%); on a 19-inch wheel: 275/55R19 (-1.0%), 265/55R19 (-2.4%). The percentage is the diameter change, which is also your speedometer error.
Can I use 295/45R20 instead of 285/50R20?
Dimensionally yes: 295/45R20 is 2.5% shorter (-20 mm) and 10 mm wider, and at a true 60 mph the speedometer would read 61.5 mph. The extra width needs about 0.20 in of clearance on each side of the tire and a rim width that suits 295 mm.