GarageCalc What 215/40R18 means
215 is the section width in millimeters (8.5 in). 40 is the aspect ratio: the sidewall is 40 percent of the width, so 215 × 0.40 = 86 mm (3.4 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 × 86 = 629 mm, or 24.8 in.
A full marking adds a load index and speed rating after the size, for example 215/40R18 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 215/40R18
Going up one wheel size, the closest match is 225/35R19 at +1.7% diameter with a 79 mm sidewall instead of 86 mm. Going down one size, 215/45R17 is -0.6% with a taller 97 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.
Frequently asked questions
How tall is a 215/40R18 tire?
24.8 inches (629 mm) in overall diameter: the 18-inch wheel (457 mm) plus two 86 mm sidewalls. Real tires land within about 1 to 2 percent of that depending on brand, tread depth and pressure.
What is 215/40R18 in inches?
24.8 x 8.5 R18: 24.8 inches tall, 8.5 inches wide, on an 18-inch wheel. The sidewall is 3.4 inches and the circumference is 77.8 inches, so the tire turns 814 times per mile.
What tire sizes are equivalent to 215/40R18?
Within 3 percent of the same diameter on the same 18-inch wheel: 235/35R18 (-1.2%), 225/40R18 (+1.3%), 225/35R18 (-2.3%); on a 19-inch wheel: 225/35R19 (+1.7%), 235/35R19 (+2.8%); on a 17-inch wheel: 215/45R17 (-0.6%), 225/45R17 (+0.8%). The percentage is the diameter change, which is also your speedometer error.
Can I use 225/40R18 instead of 215/40R18?
Dimensionally yes: 225/40R18 is 1.3% taller (+8 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 225 mm.