GarageCalc What 245/70R18 means
245 is the section width in millimeters (9.6 in). 70 is the aspect ratio: the sidewall is 70 percent of the width, so 245 × 0.70 = 172 mm (6.8 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 × 172 = 800 mm, or 31.5 in.
A full marking adds a load index and speed rating after the size, for example 245/70R18 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 245/70R18
No common 19-inch size stays within 3 percent of 31.5 in. Going down one size, 245/75R17 is -0.1% with a taller 184 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 245/70R18 tire?
31.5 inches (800 mm) in overall diameter: the 18-inch wheel (457 mm) plus two 172 mm sidewalls. Real tires land within about 1 to 2 percent of that depending on brand, tread depth and pressure.
What is 245/70R18 in inches?
31.5 x 9.6 R18: 31.5 inches tall, 9.6 inches wide, on an 18-inch wheel. The sidewall is 6.8 inches and the circumference is 99.0 inches, so the tire turns 640 times per mile.
What tire sizes are equivalent to 245/70R18?
Within 3 percent of the same diameter on the same 18-inch wheel: 265/65R18 (+0.2%), 255/65R18 (-1.4%), 255/70R18 (+1.7%); on a 17-inch wheel: 245/75R17 (-0.1%), 265/70R17 (+0.3%). The percentage is the diameter change, which is also your speedometer error.
Can I use 255/65R18 instead of 245/70R18?
Dimensionally yes: 255/65R18 is 1.4% shorter (-12 mm) and 10 mm wider, and at a true 60 mph the speedometer would read 60.9 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.