GarageCalc What 155/80R12 means
155 is the section width in millimeters (6.1 in). 80 is the aspect ratio: the sidewall is 80 percent of the width, so 155 × 0.80 = 124 mm (4.9 in). R is radial construction, and 12 is the wheel diameter in inches (305 mm). Stack two sidewalls on the wheel and you get the overall diameter: 305 + 2 × 124 = 553 mm, or 21.8 in.
A full marking adds a load index and speed rating after the size, for example 155/80R12 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 155/80R12
Going up one wheel size, the closest match is 175/65R13 at +0.9% diameter with a 114 mm sidewall instead of 124 mm. No common 11-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 155/80R12 tire?
21.8 inches (553 mm) in overall diameter: the 12-inch wheel (305 mm) plus two 124 mm sidewalls. Real tires land within about 1 to 2 percent of that depending on brand, tread depth and pressure.
What is 155/80R12 in inches?
21.8 x 6.1 R12: 21.8 inches tall, 6.1 inches wide, on a 12-inch wheel. The sidewall is 4.9 inches and the circumference is 68.4 inches, so the tire turns 927 times per mile.
What tire sizes are equivalent to 155/80R12?
Within 3 percent of the same diameter on the same 12-inch wheel: 145/80R12 (-2.9%); on a 13-inch wheel: 175/65R13 (+0.9%), 155/70R13 (-1.0%). The percentage is the diameter change, which is also your speedometer error.