GarageCalc What 265/40R18 means
265 is the section width in millimeters (10.4 in). 40 is the aspect ratio: the sidewall is 40 percent of the width, so 265 × 0.40 = 106 mm (4.2 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 × 106 = 669 mm, or 26.3 in.
A full marking adds a load index and speed rating after the size, for example 265/40R18 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 265/40R18
Going up one wheel size, the closest match is 265/35R19 at -0.2% diameter with a 93 mm sidewall instead of 106 mm. Going down one size, 255/45R17 is -1.2% 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 265/40R18 tire?
26.3 inches (669 mm) in overall diameter: the 18-inch wheel (457 mm) plus two 106 mm sidewalls. Real tires land within about 1 to 2 percent of that depending on brand, tread depth and pressure.
What is 265/40R18 in inches?
26.3 x 10.4 R18: 26.3 inches tall, 10.4 inches wide, on an 18-inch wheel. The sidewall is 4.2 inches and the circumference is 82.8 inches, so the tire turns 765 times per mile.
What tire sizes are equivalent to 265/40R18?
Within 3 percent of the same diameter on the same 18-inch wheel: 255/40R18 (-1.2%), 275/40R18 (+1.2%), 245/45R18 (+1.3%); on a 19-inch wheel: 265/35R19 (-0.2%), 275/35R19 (+0.9%); on a 17-inch wheel: 255/45R17 (-1.2%), 285/40R17 (-1.4%). The percentage is the diameter change, which is also your speedometer error.
Can I use 275/40R18 instead of 265/40R18?
Dimensionally yes: 275/40R18 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 275 mm.