GarageCalc What 285/65R18 means
285 is the section width in millimeters (11.2 in). 65 is the aspect ratio: the sidewall is 65 percent of the width, so 285 × 0.65 = 185 mm (7.3 in). R is radial construction, and 18 is the wheel diameter in inches (457 mm). Stack two sidewalls on the wheel and you get the overall diameter: 457 + 2 × 185 = 828 mm, or 32.6 in.
A full marking adds a load index and speed rating after the size, for example 285/65R18 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 285/65R18
No common 19-inch size stays within 3 percent of 32.6 in. Going down one size, 285/70R17 is +0.4% with a taller 200 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 285/65R18 tire?
32.6 inches (828 mm) in overall diameter: the 18-inch wheel (457 mm) plus two 185 mm sidewalls. Real tires land within about 1 to 2 percent of that depending on brand, tread depth and pressure.
What is 285/65R18 in inches?
32.6 x 11.2 R18: 32.6 inches tall, 11.2 inches wide, on an 18-inch wheel. The sidewall is 7.3 inches and the circumference is 102.4 inches, so the tire turns 619 times per mile.
What tire sizes are equivalent to 285/65R18?
Within 3 percent of the same diameter on the same 18-inch wheel: 265/70R18 (+0.1%), 275/65R18 (-1.6%), 275/70R18 (+1.8%); on a 17-inch wheel: 285/70R17 (+0.4%), 275/70R17 (-1.3%). The percentage is the diameter change, which is also your speedometer error.