GarageCalc What 255/40R18 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 18 is the wheel diameter in inches (457 mm). Stack two sidewalls on the wheel and you get the overall diameter: 457 + 2 × 102 = 661 mm, or 26.0 in.
A full marking adds a load index and speed rating after the size, for example 255/40R18 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 255/40R18
Going up one wheel size, the closest match is 255/35R19 at -0.0% diameter with a 89 mm sidewall instead of 102 mm. Going down one size, 255/45R17 is +0.0% with a taller 115 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 255/40R18 tire?
26.0 inches (661 mm) in overall diameter: the 18-inch wheel (457 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 255/40R18 in inches?
26.0 x 10.0 R18: 26.0 inches tall, 10.0 inches wide, on an 18-inch wheel. The sidewall is 4.0 inches and the circumference is 81.8 inches, so the tire turns 775 times per mile.
What tire sizes are equivalent to 255/40R18?
Within 3 percent of the same diameter on the same 18-inch wheel: 235/45R18 (+1.1%), 245/40R18 (-1.2%), 265/40R18 (+1.2%); on a 19-inch wheel: 255/35R19 (-0.0%), 265/35R19 (+1.0%); on a 17-inch wheel: 255/45R17 (+0.0%), 235/50R17 (+0.8%). The percentage is the diameter change, which is also your speedometer error.
Can I use 265/40R18 instead of 255/40R18?
Dimensionally yes: 265/40R18 is 1.2% taller (+8 mm) and 10 mm wider, and at a true 60 mph the speedometer would read 59.3 mph. The extra width needs about 0.20 in of clearance on each side of the tire and a rim width that suits 265 mm.