GarageCalc What 285/40R18 means
285 is the section width in millimeters (11.2 in). 40 is the aspect ratio: the sidewall is 40 percent of the width, so 285 × 0.40 = 114 mm (4.5 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 × 114 = 685 mm, or 27.0 in.
A full marking adds a load index and speed rating after the size, for example 285/40R18 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 285/40R18
Going up one wheel size, the closest match is 285/35R19 at -0.5% diameter with a 100 mm sidewall instead of 114 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 285/40R18 tire?
27.0 inches (685 mm) in overall diameter: the 18-inch wheel (457 mm) plus two 114 mm sidewalls. Real tires land within about 1 to 2 percent of that depending on brand, tread depth and pressure.
What is 285/40R18 in inches?
27.0 x 11.2 R18: 27.0 inches tall, 11.2 inches wide, on an 18-inch wheel. The sidewall is 4.5 inches and the circumference is 84.7 inches, so the tire turns 748 times per mile.
What tire sizes are equivalent to 285/40R18?
Within 3 percent of the same diameter on the same 18-inch wheel: 275/40R18 (-1.2%), 305/35R18 (-2.1%), 265/40R18 (-2.3%); on a 19-inch wheel: 285/35R19 (-0.5%), 295/35R19 (+0.6%). The percentage is the diameter change, which is also your speedometer error.
Can I use 305/35R18 instead of 285/40R18?
Dimensionally yes: 305/35R18 is 2.1% shorter (-15 mm) and 20 mm wider, and at a true 60 mph the speedometer would read 61.3 mph. The extra width needs about 0.39 in of clearance on each side of the tire and a rim width that suits 305 mm.