GarageCalc What 285/70R18 means
285 is the section width in millimeters (11.2 in). 70 is the aspect ratio: the sidewall is 70 percent of the width, so 285 × 0.70 = 200 mm (7.9 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 × 200 = 856 mm, or 33.7 in.
A full marking adds a load index and speed rating after the size, for example 285/70R18 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 285/70R18
No common 19-inch size stays within 3 percent of 33.7 in. Going down one size, 285/75R17 is +0.4% with a taller 214 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 285/70R18 tire?
33.7 inches (856 mm) in overall diameter: the 18-inch wheel (457 mm) plus two 200 mm sidewalls. Real tires land within about 1 to 2 percent of that depending on brand, tread depth and pressure.
What is 285/70R18 in inches?
33.7 x 11.2 R18: 33.7 inches tall, 11.2 inches wide, on an 18-inch wheel. The sidewall is 7.9 inches and the circumference is 105.9 inches, so the tire turns 598 times per mile.
What tire sizes are equivalent to 285/70R18?
Within 3 percent of the same diameter on the same 18-inch wheel: 275/70R18 (-1.6%), 295/70R18 (+1.6%); on a 17-inch wheel: 285/75R17 (+0.4%), 295/70R17 (-1.3%). The percentage is the diameter change, which is also your speedometer error.
Can I use 295/70R18 instead of 285/70R18?
Dimensionally yes: 295/70R18 is 1.6% taller (+14 mm) and 10 mm wider, and at a true 60 mph the speedometer would read 59.0 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.