GarageCalc What 225/55R18 means
225 is the section width in millimeters (8.9 in). 55 is the aspect ratio: the sidewall is 55 percent of the width, so 225 × 0.55 = 124 mm (4.9 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 × 124 = 705 mm, or 27.7 in.
A full marking adds a load index and speed rating after the size, for example 225/55R18 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 225/55R18
Going up one wheel size, the closest match is 245/45R19 at -0.2% diameter with a 110 mm sidewall instead of 124 mm. Going down one size, 225/60R17 is -0.4% with a taller 135 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 225/55R18 tire?
27.7 inches (705 mm) in overall diameter: the 18-inch wheel (457 mm) plus two 124 mm sidewalls. Real tires land within about 1 to 2 percent of that depending on brand, tread depth and pressure.
What is 225/55R18 in inches?
27.7 x 8.9 R18: 27.7 inches tall, 8.9 inches wide, on an 18-inch wheel. The sidewall is 4.9 inches and the circumference is 87.2 inches, so the tire turns 727 times per mile.
What tire sizes are equivalent to 225/55R18?
Within 3 percent of the same diameter on the same 18-inch wheel: 245/50R18 (-0.4%), 215/55R18 (-1.6%), 235/55R18 (+1.6%); on a 19-inch wheel: 245/45R19 (-0.2%), 235/45R19 (-1.5%); on a 17-inch wheel: 225/60R17 (-0.4%), 245/55R17 (-0.5%). The percentage is the diameter change, which is also your speedometer error.
Can I use 235/55R18 instead of 225/55R18?
Dimensionally yes: 235/55R18 is 1.6% 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 235 mm.