GarageCalc What 255/55R18 means
255 is the section width in millimeters (10.0 in). 55 is the aspect ratio: the sidewall is 55 percent of the width, so 255 × 0.55 = 140 mm (5.5 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 × 140 = 738 mm, or 29.0 in.
A full marking adds a load index and speed rating after the size, for example 255/55R18 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 255/55R18
Similar diameter does not establish interchangeability. Follow the vehicle manufacturer's approved sizes; see Michelin's tire-size guidance.
Going up one wheel size, the closest match is 255/50R19 at -0.0% diameter with a 128 mm sidewall instead of 140 mm. Going down one size, 255/60R17 is +0.0% with a taller 153 mm sidewall for more comfort and pothole margin. The 3 percent cutoff is a comparison filter, not a fitment approval. Use vehicle-approved sizes and check rim width, load and speed ratings, and clearances. The comparison calculator shows the full speedometer table for any pair.
Frequently asked questions
How tall is a 255/55R18 tire?
29.0 inches (738 mm) in overall diameter: the 18-inch wheel (457 mm) plus two 140 mm sidewalls. Actual dimensions depend on the tire model, tread depth, pressure and load; check the tire manufacturer specifications.
What is 255/55R18 in inches?
29.0 x 10.0 R18: 29.0 inches tall, 10.0 inches wide, on an 18-inch wheel. The sidewall is 5.5 inches and the circumference is 91.2 inches, so the tire turns 694 times per mile.
What tire sizes are equivalent to 255/55R18?
Within 3 percent of the same diameter on the same 18-inch wheel: 235/60R18 (+0.2%), 245/55R18 (-1.5%), 245/60R18 (+1.8%); on a 19-inch wheel: 255/50R19 (-0.0%), 235/55R19 (+0.5%); on a 17-inch wheel: 255/60R17 (+0.0%), 235/65R17 (-0.1%). The percentage is the diameter change, which is also your speedometer error.