GarageCalc What 245/40R18 means
245 is the section width in millimeters (9.6 in). 40 is the aspect ratio: the sidewall is 40 percent of the width, so 245 × 0.40 = 98 mm (3.9 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 × 98 = 653 mm, or 25.7 in.
A full marking adds a load index and speed rating after the size, for example 245/40R18 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 245/40R18
Going up one wheel size, the closest match is 245/35R19 at +0.1% diameter with a 86 mm sidewall instead of 98 mm. Going down one size, 245/45R17 is -0.1% with a taller 110 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 245/40R18
Typical factory fitments include Subaru WRX (2022 on) and Audi A4 (B9) 18-inch trims. Tire size changes with trim and wheel option, so confirm on the door-jamb placard before buying.
Frequently asked questions
How tall is a 245/40R18 tire?
25.7 inches (653 mm) in overall diameter: the 18-inch wheel (457 mm) plus two 98 mm sidewalls. Real tires land within about 1 to 2 percent of that depending on brand, tread depth and pressure.
What is 245/40R18 in inches?
25.7 x 9.6 R18: 25.7 inches tall, 9.6 inches wide, on an 18-inch wheel. The sidewall is 3.9 inches and the circumference is 80.8 inches, so the tire turns 784 times per mile.
What tire sizes are equivalent to 245/40R18?
Within 3 percent of the same diameter on the same 18-inch wheel: 225/45R18 (+1.0%), 235/40R18 (-1.2%), 255/40R18 (+1.2%); on a 19-inch wheel: 245/35R19 (+0.1%), 235/35R19 (-0.9%); on a 17-inch wheel: 245/45R17 (-0.1%), 225/50R17 (+0.6%). The percentage is the diameter change, which is also your speedometer error.
Can I use 255/40R18 instead of 245/40R18?
Dimensionally yes: 255/40R18 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 255 mm.