GarageCalc What 235/45R18 means
235 is the section width in millimeters (9.3 in). 45 is the aspect ratio: the sidewall is 45 percent of the width, so 235 × 0.45 = 106 mm (4.2 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 × 106 = 669 mm, or 26.3 in.
A full marking adds a load index and speed rating after the size, for example 235/45R18 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 235/45R18
Going up one wheel size, the closest match is 235/40R19 at +0.3% diameter with a 94 mm sidewall instead of 106 mm. Going down one size, 215/55R17 is -0.1% with a taller 118 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 235/45R18
Typical factory fitments include Toyota Camry SE and XSE (2018 on), Honda Accord Sport (2013-2017), Tesla Model 3 (18-inch), Hyundai Sonata Sport and Lexus ES F Sport. Tire size changes with trim and wheel option, so confirm on the door-jamb placard before buying.
Frequently asked questions
How tall is a 235/45R18 tire?
26.3 inches (669 mm) in overall diameter: the 18-inch wheel (457 mm) plus two 106 mm sidewalls. Real tires land within about 1 to 2 percent of that depending on brand, tread depth and pressure.
What is 235/45R18 in inches?
26.3 x 9.3 R18: 26.3 inches tall, 9.3 inches wide, on an 18-inch wheel. The sidewall is 4.2 inches and the circumference is 82.7 inches, so the tire turns 766 times per mile.
What tire sizes are equivalent to 235/45R18?
Within 3 percent of the same diameter on the same 18-inch wheel: 215/50R18 (+0.5%), 255/40R18 (-1.1%), 225/45R18 (-1.3%); on a 19-inch wheel: 235/40R19 (+0.3%), 225/40R19 (-0.9%); on a 17-inch wheel: 215/55R17 (-0.1%), 235/50R17 (-0.3%). The percentage is the diameter change, which is also your speedometer error.
Can I use 245/45R18 instead of 235/45R18?
Dimensionally yes: 245/45R18 is 1.3% taller (+9 mm) and 10 mm wider, and at a true 60 mph the speedometer would read 59.2 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.