GarageCalc What 255/45R18 means
255 is the section width in millimeters (10.0 in). 45 is the aspect ratio: the sidewall is 45 percent of the width, so 255 × 0.45 = 115 mm (4.5 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 × 115 = 687 mm, or 27.0 in.
A full marking adds a load index and speed rating after the size, for example 255/45R18 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 255/45R18
Going up one wheel size, the closest match is 255/40R19 at -0.0% diameter with a 102 mm sidewall instead of 115 mm. Going down one size, 235/55R17 is +0.5% with a taller 129 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 255/45R18 tire?
27.0 inches (687 mm) in overall diameter: the 18-inch wheel (457 mm) plus two 115 mm sidewalls. Real tires land within about 1 to 2 percent of that depending on brand, tread depth and pressure.
What is 255/45R18 in inches?
27.0 x 10.0 R18: 27.0 inches tall, 10.0 inches wide, on an 18-inch wheel. The sidewall is 4.5 inches and the circumference is 84.9 inches, so the tire turns 746 times per mile.
What tire sizes are equivalent to 255/45R18?
Within 3 percent of the same diameter on the same 18-inch wheel: 235/50R18 (+0.8%), 245/45R18 (-1.3%), 275/40R18 (-1.4%); on a 19-inch wheel: 255/40R19 (-0.0%), 235/45R19 (+1.1%); on a 17-inch wheel: 235/55R17 (+0.5%), 245/55R17 (+2.1%). The percentage is the diameter change, which is also your speedometer error.
Can I use 265/40R18 instead of 255/45R18?
Dimensionally yes: 265/40R18 is 2.5% shorter (-18 mm) and 10 mm wider, and at a true 60 mph the speedometer would read 61.6 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.