GarageCalc What 285/30R18 means
285 is the section width in millimeters (11.2 in). 30 is the aspect ratio: the sidewall is 30 percent of the width, so 285 × 0.30 = 86 mm (3.4 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 × 86 = 628 mm, or 24.7 in.
A full marking adds a load index and speed rating after the size, for example 285/30R18 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 285/30R18
Going up one wheel size, the closest match is 265/30R19 at +2.1% diameter with a 80 mm sidewall instead of 86 mm. No common 17-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 285/30R18 tire?
24.7 inches (628 mm) in overall diameter: the 18-inch wheel (457 mm) plus two 86 mm sidewalls. Real tires land within about 1 to 2 percent of that depending on brand, tread depth and pressure.
What is 285/30R18 in inches?
24.7 x 11.2 R18: 24.7 inches tall, 11.2 inches wide, on an 18-inch wheel. The sidewall is 3.4 inches and the circumference is 77.7 inches, so the tire turns 815 times per mile.
What tire sizes are equivalent to 285/30R18?
Within 3 percent of the same diameter on the same 18-inch wheel: 295/30R18 (+1.0%), 265/35R18 (+2.3%); on a 19-inch wheel: 265/30R19 (+2.1%). The percentage is the diameter change, which is also your speedometer error.
Can I use 295/30R18 instead of 285/30R18?
Dimensionally yes: 295/30R18 is 1.0% taller (+6 mm) and 10 mm wider, and at a true 60 mph the speedometer would read 59.4 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.