GarageCalc What 255/65R18 means
255 is the section width in millimeters (10.0 in). 65 is the aspect ratio: the sidewall is 65 percent of the width, so 255 × 0.65 = 166 mm (6.5 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 × 166 = 789 mm, or 31.1 in.
A full marking adds a load index and speed rating after the size, for example 255/65R18 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 255/65R18
Going up one wheel size, the closest match is 275/55R19 at -0.5% diameter with a 151 mm sidewall instead of 166 mm. Going down one size, 255/70R17 is +0.0% with a taller 179 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 255/65R18 tire?
31.1 inches (789 mm) in overall diameter: the 18-inch wheel (457 mm) plus two 166 mm sidewalls. Real tires land within about 1 to 2 percent of that depending on brand, tread depth and pressure.
What is 255/65R18 in inches?
31.1 x 10.0 R18: 31.1 inches tall, 10.0 inches wide, on an 18-inch wheel. The sidewall is 6.5 inches and the circumference is 97.6 inches, so the tire turns 650 times per mile.
What tire sizes are equivalent to 255/65R18?
Within 3 percent of the same diameter on the same 18-inch wheel: 245/70R18 (+1.5%), 245/65R18 (-1.6%), 265/65R18 (+1.6%); on a 19-inch wheel: 275/55R19 (-0.5%), 265/55R19 (-1.9%); on a 17-inch wheel: 255/70R17 (+0.0%), 275/65R17 (+0.1%). The percentage is the diameter change, which is also your speedometer error.
Can I use 265/65R18 instead of 255/65R18?
Dimensionally yes: 265/65R18 is 1.6% taller (+13 mm) and 10 mm wider, and at a true 60 mph the speedometer would read 59.0 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.