GarageCalc What 205/40R17 means
205 is the section width in millimeters (8.1 in). 40 is the aspect ratio: the sidewall is 40 percent of the width, so 205 × 0.40 = 82 mm (3.2 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 × 82 = 596 mm, or 23.5 in.
A full marking adds a load index and speed rating after the size, for example 205/40R17 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 205/40R17
Going up one wheel size, the closest match is 215/35R18 at +2.0% diameter with a 75 mm sidewall instead of 82 mm. Going down one size, 185/50R16 is -0.7% with a taller 93 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 205/40R17 tire?
23.5 inches (596 mm) in overall diameter: the 17-inch wheel (432 mm) plus two 82 mm sidewalls. Real tires land within about 1 to 2 percent of that depending on brand, tread depth and pressure.
What is 205/40R17 in inches?
23.5 x 8.1 R17: 23.5 inches tall, 8.1 inches wide, on a 17-inch wheel. The sidewall is 3.2 inches and the circumference is 73.7 inches, so the tire turns 860 times per mile.
What tire sizes are equivalent to 205/40R17?
Within 3 percent of the same diameter on the same 17-inch wheel: 215/40R17 (+1.3%), 225/40R17 (+2.7%); on an 18-inch wheel: 215/35R18 (+2.0%); on a 16-inch wheel: 185/50R16 (-0.7%), 205/45R16 (-0.8%). The percentage is the diameter change, which is also your speedometer error.
Can I use 215/40R17 instead of 205/40R17?
Dimensionally yes: 215/40R17 is 1.3% taller (+8 mm) and 10 mm wider, and at a true 60 mph the speedometer would read 59.2 mph. The extra width needs about 0.20 in of clearance on each side of the tire and a rim width that suits 215 mm.