GarageCalc What 245/65R18 means
245 is the section width in millimeters (9.6 in). 65 is the aspect ratio: the sidewall is 65 percent of the width, so 245 × 0.65 = 159 mm (6.3 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 × 159 = 776 mm, or 30.5 in.
A full marking adds a load index and speed rating after the size, for example 245/65R18 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 245/65R18
Going up one wheel size, the closest match is 265/55R19 at -0.2% diameter with a 146 mm sidewall instead of 159 mm. Going down one size, 265/65R17 is +0.1% with a taller 172 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 245/65R18 tire?
30.5 inches (776 mm) in overall diameter: the 18-inch wheel (457 mm) plus two 159 mm sidewalls. Real tires land within about 1 to 2 percent of that depending on brand, tread depth and pressure.
What is 245/65R18 in inches?
30.5 x 9.6 R18: 30.5 inches tall, 9.6 inches wide, on an 18-inch wheel. The sidewall is 6.3 inches and the circumference is 95.9 inches, so the tire turns 660 times per mile.
What tire sizes are equivalent to 245/65R18?
Within 3 percent of the same diameter on the same 18-inch wheel: 265/60R18 (-0.1%), 255/60R18 (-1.6%), 235/65R18 (-1.7%); on a 19-inch wheel: 265/55R19 (-0.2%), 255/55R19 (-1.6%); on a 17-inch wheel: 265/65R17 (+0.1%), 245/70R17 (-0.1%). The percentage is the diameter change, which is also your speedometer error.
Can I use 255/60R18 instead of 245/65R18?
Dimensionally yes: 255/60R18 is 1.6% shorter (-13 mm) and 10 mm wider, and at a true 60 mph the speedometer would read 61.0 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.