GarageCalc What 225/40R18 means
225 is the section width in millimeters (8.9 in). 40 is the aspect ratio: the sidewall is 40 percent of the width, so 225 × 0.40 = 90 mm (3.5 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 × 90 = 637 mm, or 25.1 in.
A full marking adds a load index and speed rating after the size, for example 225/40R18 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 225/40R18
Going up one wheel size, the closest match is 225/35R19 at +0.5% diameter with a 79 mm sidewall instead of 90 mm. Going down one size, 205/50R17 is -0.1% with a taller 103 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 225/40R18
Typical factory fitments include VW Golf GTI (Mk7), Audi A3 18-inch trims, Mercedes-Benz CLA, Kia Forte GT and Toyota Corolla SE and XSE (2020 on). Tire size changes with trim and wheel option, so confirm on the door-jamb placard before buying.
Frequently asked questions
How tall is a 225/40R18 tire?
25.1 inches (637 mm) in overall diameter: the 18-inch wheel (457 mm) plus two 90 mm sidewalls. Real tires land within about 1 to 2 percent of that depending on brand, tread depth and pressure.
What is 225/40R18 in inches?
25.1 x 8.9 R18: 25.1 inches tall, 8.9 inches wide, on an 18-inch wheel. The sidewall is 3.5 inches and the circumference is 78.8 inches, so the tire turns 804 times per mile.
What tire sizes are equivalent to 225/40R18?
Within 3 percent of the same diameter on the same 18-inch wheel: 215/40R18 (-1.3%), 235/40R18 (+1.3%), 245/35R18 (-1.3%); on a 19-inch wheel: 225/35R19 (+0.5%), 235/35R19 (+1.6%); on a 17-inch wheel: 205/50R17 (-0.1%), 225/45R17 (-0.5%). The percentage is the diameter change, which is also your speedometer error.
Can I use 235/40R18 instead of 225/40R18?
Dimensionally yes: 235/40R18 is 1.3% 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 235 mm.