GarageCalc What 245/55R18 means
245 is the section width in millimeters (9.6 in). 55 is the aspect ratio: the sidewall is 55 percent of the width, so 245 × 0.55 = 135 mm (5.3 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 × 135 = 727 mm, or 28.6 in.
A full marking adds a load index and speed rating after the size, for example 245/55R18 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 245/55R18
Going up one wheel size, the closest match is 245/50R19 at +0.1% diameter with a 123 mm sidewall instead of 135 mm. Going down one size, 225/65R17 is -0.3% with a taller 146 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 245/55R18 tire?
28.6 inches (727 mm) in overall diameter: the 18-inch wheel (457 mm) plus two 135 mm sidewalls. Real tires land within about 1 to 2 percent of that depending on brand, tread depth and pressure.
What is 245/55R18 in inches?
28.6 x 9.6 R18: 28.6 inches tall, 9.6 inches wide, on an 18-inch wheel. The sidewall is 5.3 inches and the circumference is 89.9 inches, so the tire turns 705 times per mile.
What tire sizes are equivalent to 245/55R18?
Within 3 percent of the same diameter on the same 18-inch wheel: 225/60R18 (+0.1%), 235/55R18 (-1.5%), 255/55R18 (+1.5%); on a 19-inch wheel: 245/50R19 (+0.1%), 225/55R19 (+0.5%); on a 17-inch wheel: 225/65R17 (-0.3%), 235/65R17 (+1.5%). The percentage is the diameter change, which is also your speedometer error.
Can I use 255/55R18 instead of 245/55R18?
Dimensionally yes: 255/55R18 is 1.5% taller (+11 mm) and 10 mm wider, and at a true 60 mph the speedometer would read 59.1 mph. The extra width needs about 0.20 in of clearance on each side of the tire and a rim width that suits 255 mm.