GarageCalc What 255/70R15 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 15 is the wheel diameter in inches (381 mm). Stack two sidewalls on the wheel and you get the overall diameter: 381 + 2 × 179 = 738 mm, or 29.1 in.
A full marking adds a load index and speed rating after the size, for example 255/70R15 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 255/70R15
Going up one wheel size, the closest match is 255/65R16 at -0.0% diameter with a 166 mm sidewall instead of 179 mm. No common 14-inch size stays within 3 percent either. 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 255/70R15 tire?
29.1 inches (738 mm) in overall diameter: the 15-inch wheel (381 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 255/70R15 in inches?
29.1 x 10.0 R15: 29.1 inches tall, 10.0 inches wide, on a 15-inch wheel. The sidewall is 7.0 inches and the circumference is 91.3 inches, so the tire turns 694 times per mile.
What tire sizes are equivalent to 255/70R15?
Within 3 percent of the same diameter on the same 15-inch wheel: 235/75R15 (-0.6%), 265/70R15 (+1.9%); on a 16-inch wheel: 255/65R16 (-0.0%), 235/70R16 (-0.4%). The percentage is the diameter change, which is also your speedometer error.
Can I use 265/70R15 instead of 255/70R15?
Dimensionally yes: 265/70R15 is 1.9% taller (+14 mm) and 10 mm wider, and at a true 60 mph the speedometer would read 58.9 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.