GarageCalc What 225/45R18 means
225 is the section width in millimeters (8.9 in). 45 is the aspect ratio: the sidewall is 45 percent of the width, so 225 × 0.45 = 101 mm (4.0 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 × 101 = 660 mm, or 26.0 in.
A full marking adds a load index and speed rating after the size, for example 225/45R18 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 225/45R18
Going up one wheel size, the closest match is 225/40R19 at +0.4% diameter with a 90 mm sidewall instead of 101 mm. Going down one size, 205/55R17 is -0.4% with a taller 113 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 225/45R18 tire?
26.0 inches (660 mm) in overall diameter: the 18-inch wheel (457 mm) plus two 101 mm sidewalls. Real tires land within about 1 to 2 percent of that depending on brand, tread depth and pressure.
What is 225/45R18 in inches?
26.0 x 8.9 R18: 26.0 inches tall, 8.9 inches wide, on an 18-inch wheel. The sidewall is 4.0 inches and the circumference is 81.6 inches, so the tire turns 777 times per mile.
What tire sizes are equivalent to 225/45R18?
Within 3 percent of the same diameter on the same 18-inch wheel: 245/40R18 (-1.0%), 215/45R18 (-1.4%), 235/45R18 (+1.4%); on a 19-inch wheel: 225/40R19 (+0.4%), 245/35R19 (-0.8%); on a 17-inch wheel: 205/55R17 (-0.4%), 225/50R17 (-0.4%). The percentage is the diameter change, which is also your speedometer error.
Can I use 235/45R18 instead of 225/45R18?
Dimensionally yes: 235/45R18 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 235 mm.