GarageCalc What 255/70R18 means
255 is the section width in millimeters (10.0 in). 70 is the aspect ratio: the sidewall is 70 percent of the width, so 255 × 0.70 = 179 mm (7.0 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 × 179 = 814 mm, or 32.1 in.
A full marking adds a load index and speed rating after the size, for example 255/70R18 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 255/70R18
No common 19-inch size stays within 3 percent of 32.1 in. Going down one size, 255/75R17 is +0.0% with a taller 191 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.
Which cars come with 255/70R18
Typical factory fitments include Jeep Wrangler Sahara (JK and JL) and Toyota Tundra SR5 (2007-2021). Tire size changes with trim and wheel option, so confirm on the door-jamb placard before buying.
Frequently asked questions
How tall is a 255/70R18 tire?
32.1 inches (814 mm) in overall diameter: the 18-inch wheel (457 mm) plus two 179 mm sidewalls. Real tires land within about 1 to 2 percent of that depending on brand, tread depth and pressure.
What is 255/70R18 in inches?
32.1 x 10.0 R18: 32.1 inches tall, 10.0 inches wide, on an 18-inch wheel. The sidewall is 7.0 inches and the circumference is 100.7 inches, so the tire turns 629 times per mile.
What tire sizes are equivalent to 255/70R18?
Within 3 percent of the same diameter on the same 18-inch wheel: 275/65R18 (+0.1%), 265/65R18 (-1.5%), 245/70R18 (-1.7%); on a 17-inch wheel: 255/75R17 (+0.0%), 275/70R17 (+0.3%). The percentage is the diameter change, which is also your speedometer error.
Can I use 265/65R18 instead of 255/70R18?
Dimensionally yes: 265/65R18 is 1.5% shorter (-13 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 265 mm.