GarageCalc What 255/40R17 means
255 is the section width in millimeters (10.0 in). 40 is the aspect ratio: the sidewall is 40 percent of the width, so 255 × 0.40 = 102 mm (4.0 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 × 102 = 636 mm, or 25.0 in.
A full marking adds a load index and speed rating after the size, for example 255/40R17 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 255/40R17
Going up one wheel size, the closest match is 255/35R18 at -0.0% diameter with a 89 mm sidewall instead of 102 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 255/40R17 tire?
25.0 inches (636 mm) in overall diameter: the 17-inch wheel (432 mm) plus two 102 mm sidewalls. Real tires land within about 1 to 2 percent of that depending on brand, tread depth and pressure.
What is 255/40R17 in inches?
25.0 x 10.0 R17: 25.0 inches tall, 10.0 inches wide, on a 17-inch wheel. The sidewall is 4.0 inches and the circumference is 78.6 inches, so the tire turns 806 times per mile.
What tire sizes are equivalent to 255/40R17?
Within 3 percent of the same diameter on the same 17-inch wheel: 235/45R17 (+1.2%), 245/40R17 (-1.3%), 235/40R17 (-2.5%); on an 18-inch wheel: 255/35R18 (-0.0%), 265/35R18 (+1.1%). The percentage is the diameter change, which is also your speedometer error.
Can I use 275/40R17 instead of 255/40R17?
Dimensionally yes: 275/40R17 is 2.5% taller (+16 mm) and 20 mm wider, and at a true 60 mph the speedometer would read 58.5 mph. The extra width needs about 0.39 in of clearance on each side of the tire and a rim width that suits 275 mm.