GarageCalc What 235/40R18 means
235 is the section width in millimeters (9.3 in). 40 is the aspect ratio: the sidewall is 40 percent of the width, so 235 × 0.40 = 94 mm (3.7 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 × 94 = 645 mm, or 25.4 in.
A full marking adds a load index and speed rating after the size, for example 235/40R18 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 235/40R18
Going up one wheel size, the closest match is 235/35R19 at +0.3% diameter with a 82 mm sidewall instead of 94 mm. Going down one size, 215/50R17 is +0.2% with a taller 108 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.
Which cars come with 235/40R18
Typical factory fitments include Honda Civic Si (2017 on), Honda Civic Sport hatchback (2017 on) and Ford Focus ST. Tire size changes with trim and wheel option, so confirm on the door-jamb placard before buying.
Frequently asked questions
How tall is a 235/40R18 tire?
25.4 inches (645 mm) in overall diameter: the 18-inch wheel (457 mm) plus two 94 mm sidewalls. Real tires land within about 1 to 2 percent of that depending on brand, tread depth and pressure.
What is 235/40R18 in inches?
25.4 x 9.3 R18: 25.4 inches tall, 9.3 inches wide, on an 18-inch wheel. The sidewall is 3.7 inches and the circumference is 79.8 inches, so the tire turns 794 times per mile.
What tire sizes are equivalent to 235/40R18?
Within 3 percent of the same diameter on the same 18-inch wheel: 215/45R18 (+0.9%), 225/40R18 (-1.2%), 245/40R18 (+1.2%); on a 19-inch wheel: 235/35R19 (+0.3%), 225/35R19 (-0.8%); on a 17-inch wheel: 215/50R17 (+0.2%), 235/45R17 (-0.3%). The percentage is the diameter change, which is also your speedometer error.
Can I use 245/40R18 instead of 235/40R18?
Dimensionally yes: 245/40R18 is 1.2% taller (+8 mm) and 10 mm wider, and at a true 60 mph the speedometer would read 59.3 mph. The extra width needs about 0.20 in of clearance on each side of the tire and a rim width that suits 245 mm.