GarageCalc What 215/45R18 means
215 is the section width in millimeters (8.5 in). 45 is the aspect ratio: the sidewall is 45 percent of the width, so 215 × 0.45 = 97 mm (3.8 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 × 97 = 651 mm, or 25.6 in.
A full marking adds a load index and speed rating after the size, for example 215/45R18 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 215/45R18
Going up one wheel size, the closest match is 235/35R19 at -0.6% diameter with a 82 mm sidewall instead of 97 mm. Going down one size, 215/50R17 is -0.6% 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.
Frequently asked questions
How tall is a 215/45R18 tire?
25.6 inches (651 mm) in overall diameter: the 18-inch wheel (457 mm) plus two 97 mm sidewalls. Real tires land within about 1 to 2 percent of that depending on brand, tread depth and pressure.
What is 215/45R18 in inches?
25.6 x 8.5 R18: 25.6 inches tall, 8.5 inches wide, on an 18-inch wheel. The sidewall is 3.8 inches and the circumference is 80.5 inches, so the tire turns 787 times per mile.
What tire sizes are equivalent to 215/45R18?
Within 3 percent of the same diameter on the same 18-inch wheel: 235/40R18 (-0.8%), 225/45R18 (+1.4%), 225/40R18 (-2.1%); on a 19-inch wheel: 235/35R19 (-0.6%), 225/35R19 (-1.6%); on a 17-inch wheel: 215/50R17 (-0.6%), 225/50R17 (+0.9%). The percentage is the diameter change, which is also your speedometer error.
Can I use 225/45R18 instead of 215/45R18?
Dimensionally yes: 225/45R18 is 1.4% 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 225 mm.