GarageCalc What 255/70R17 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 17 is the wheel diameter in inches (432 mm). Stack two sidewalls on the wheel and you get the overall diameter: 432 + 2 × 179 = 789 mm, or 31.1 in.
A full marking adds a load index and speed rating after the size, for example 255/70R17 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 255/70R17
Going up one wheel size, the closest match is 255/65R18 at -0.0% diameter with a 166 mm sidewall instead of 179 mm. Going down one size, 275/70R16 is +0.3% with a taller 193 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 255/70R17 tire?
31.1 inches (789 mm) in overall diameter: the 17-inch wheel (432 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 255/70R17 in inches?
31.1 x 10.0 R17: 31.1 inches tall, 10.0 inches wide, on a 17-inch wheel. The sidewall is 7.0 inches and the circumference is 97.6 inches, so the tire turns 649 times per mile.
What tire sizes are equivalent to 255/70R17?
Within 3 percent of the same diameter on the same 17-inch wheel: 275/65R17 (+0.1%), 235/75R17 (-0.6%), 245/75R17 (+1.3%); on an 18-inch wheel: 255/65R18 (-0.0%), 245/70R18 (+1.4%); on a 16-inch wheel: 275/70R16 (+0.3%), 265/70R16 (-1.4%). The percentage is the diameter change, which is also your speedometer error.
Can I use 265/65R17 instead of 255/70R17?
Dimensionally yes: 265/65R17 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 265 mm.