GarageCalc What 285/55R20 means
285 is the section width in millimeters (11.2 in). 55 is the aspect ratio: the sidewall is 55 percent of the width, so 285 × 0.55 = 157 mm (6.2 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 × 157 = 822 mm, or 32.3 in.
A full marking adds a load index and speed rating after the size, for example 285/55R20 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 285/55R20
Going up one wheel size, the closest match is 275/50R21 at -1.6% diameter with a 138 mm sidewall instead of 157 mm. No common 19-inch size stays within 3 percent either. 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 285/55R20 tire?
32.3 inches (822 mm) in overall diameter: the 20-inch wheel (508 mm) plus two 157 mm sidewalls. Real tires land within about 1 to 2 percent of that depending on brand, tread depth and pressure.
What is 285/55R20 in inches?
32.3 x 11.2 R20: 32.3 inches tall, 11.2 inches wide, on a 20-inch wheel. The sidewall is 6.2 inches and the circumference is 101.6 inches, so the tire turns 624 times per mile.
What tire sizes are equivalent to 285/55R20?
Within 3 percent of the same diameter on the same 20-inch wheel: 265/60R20 (+0.5%), 305/50R20 (-1.0%), 275/55R20 (-1.3%); on a 21-inch wheel: 275/50R21 (-1.6%). The percentage is the diameter change, which is also your speedometer error.
Can I use 295/55R20 instead of 285/55R20?
Dimensionally yes: 295/55R20 is 1.3% taller (+11 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 295 mm.