GarageCalc What 235/40R17 means
235 is the section width in millimeters (9.3 in). 40 is the aspect ratio: the sidewall is 40 percent of the width, so 235 × 0.40 = 94 mm (3.7 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 × 94 = 620 mm, or 24.4 in.
A full marking adds a load index and speed rating after the size, for example 235/40R17 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 235/40R17
Going up one wheel size, the closest match is 235/35R18 at +0.3% diameter with a 82 mm sidewall instead of 94 mm. Going down one size, 225/50R16 is +1.9% with a taller 113 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 235/40R17 tire?
24.4 inches (620 mm) in overall diameter: the 17-inch wheel (432 mm) plus two 94 mm sidewalls. Real tires land within about 1 to 2 percent of that depending on brand, tread depth and pressure.
What is 235/40R17 in inches?
24.4 x 9.3 R17: 24.4 inches tall, 9.3 inches wide, on a 17-inch wheel. The sidewall is 3.7 inches and the circumference is 76.7 inches, so the tire turns 827 times per mile.
What tire sizes are equivalent to 235/40R17?
Within 3 percent of the same diameter on the same 17-inch wheel: 215/45R17 (+0.9%), 225/40R17 (-1.3%), 245/40R17 (+1.3%); on an 18-inch wheel: 235/35R18 (+0.3%), 225/35R18 (-0.8%); on a 16-inch wheel: 225/50R16 (+1.9%). The percentage is the diameter change, which is also your speedometer error.
Can I use 245/40R17 instead of 235/40R17?
Dimensionally yes: 245/40R17 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 245 mm.