GarageCalc What 285/40R17 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 17 is the wheel diameter in inches (432 mm). Stack two sidewalls on the wheel and you get the overall diameter: 432 + 2 × 114 = 660 mm, or 26.0 in.
A full marking adds a load index and speed rating after the size, for example 285/40R17 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 285/40R17
Going up one wheel size, the closest match is 285/35R18 at -0.5% diameter with a 100 mm sidewall instead of 114 mm. No common 16-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 285/40R17 tire?
26.0 inches (660 mm) in overall diameter: the 17-inch wheel (432 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 285/40R17 in inches?
26.0 x 11.2 R17: 26.0 inches tall, 11.2 inches wide, on a 17-inch wheel. The sidewall is 4.5 inches and the circumference is 81.6 inches, so the tire turns 776 times per mile.
What tire sizes are equivalent to 285/40R17?
Within 3 percent of the same diameter on the same 17-inch wheel: 275/40R17 (-1.2%); on an 18-inch wheel: 285/35R18 (-0.5%), 295/35R18 (+0.6%). The percentage is the diameter change, which is also your speedometer error.