GarageCalc What 325/65R18 means
325 is the section width in millimeters (12.8 in). 65 is the aspect ratio: the sidewall is 65 percent of the width, so 325 × 0.65 = 211 mm (8.3 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 × 211 = 880 mm, or 34.6 in.
A full marking adds a load index and speed rating after the size, for example 325/65R18 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 325/65R18
No common 19-inch size stays within 3 percent of 34.6 in. Going down one size, 315/70R17 is -0.8% with a taller 221 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 325/65R18 tire?
34.6 inches (880 mm) in overall diameter: the 18-inch wheel (457 mm) plus two 211 mm sidewalls. Real tires land within about 1 to 2 percent of that depending on brand, tread depth and pressure.
What is 325/65R18 in inches?
34.6 x 12.8 R18: 34.6 inches tall, 12.8 inches wide, on an 18-inch wheel. The sidewall is 8.3 inches and the circumference is 108.8 inches, so the tire turns 582 times per mile.
What tire sizes are equivalent to 325/65R18?
Within 3 percent of the same diameter on the same 18-inch wheel: 305/70R18 (+0.5%); on a 17-inch wheel: 315/70R17 (-0.8%). The percentage is the diameter change, which is also your speedometer error.