GarageCalc What 255/40R18 means
255 is the section width in millimeters (10.0 in). 40 is the aspect ratio: the sidewall is 40 percent of the width, so 255 × 0.40 = 102 mm (4.0 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 × 102 = 661 mm, or 26.0 in.
A full marking adds a load index and speed rating after the size, for example 255/40R18 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 255/40R18
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 255/35R19 at -0.0% diameter with a 89 mm sidewall instead of 102 mm. Going down one size, 255/45R17 is +0.0% with a taller 115 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 255/40R18 tire?
26.0 inches (661 mm) in overall diameter: the 18-inch wheel (457 mm) plus two 102 mm sidewalls. Actual dimensions depend on the tire model, tread depth, pressure and load; check the tire manufacturer specifications.
What is 255/40R18 in inches?
26.0 x 10.0 R18: 26.0 inches tall, 10.0 inches wide, on an 18-inch wheel. The sidewall is 4.0 inches and the circumference is 81.8 inches, so the tire turns 775 times per mile.
What tire sizes are equivalent to 255/40R18?
Within 3 percent of the same diameter on the same 18-inch wheel: 235/45R18 (+1.1%), 245/40R18 (-1.2%), 265/40R18 (+1.2%); on a 19-inch wheel: 255/35R19 (-0.0%), 265/35R19 (+1.0%); on a 17-inch wheel: 255/45R17 (+0.0%), 235/50R17 (+0.8%). The percentage is the diameter change, which is also your speedometer error.
Can I use 265/40R18 instead of 255/40R18?
Nominal size comparison only: 265/40R18 is 1.2% taller (+8 mm) and 10 mm wider, and at a true 60 mph the speedometer would read 59.3 mph. This does not confirm fitment. Check approved rim widths, load and speed ratings, and vehicle clearances. The nominal extra width adds about 0.20 in of clearance on each side of the tire and a rim width that suits 265 mm.