GarageCalc What 155/70R12 means
155 is the section width in millimeters (6.1 in). 70 is the aspect ratio: the sidewall is 70 percent of the width, so 155 × 0.70 = 109 mm (4.3 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 × 109 = 522 mm, or 20.5 in.
A full marking adds a load index and speed rating after the size, for example 155/70R12 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 155/70R12
Similar diameter does not establish interchangeability. Follow the vehicle manufacturer's approved sizes; see Michelin's tire-size guidance.
Going up one wheel size, the closest match is 155/65R13 at +1.9% diameter with a 101 mm sidewall instead of 109 mm. No common 11-inch size stays within 3 percent either. The 3 percent cutoff is a comparison filter, not a fitment approval. Use vehicle-approved sizes and check rim width, load and speed ratings, and clearances. The comparison calculator shows the full speedometer table for any pair.
Frequently asked questions
How tall is a 155/70R12 tire?
20.5 inches (522 mm) in overall diameter: the 12-inch wheel (305 mm) plus two 109 mm sidewalls. Actual dimensions depend on the tire model, tread depth, pressure and load; check the tire manufacturer specifications.
What is 155/70R12 in inches?
20.5 x 6.1 R12: 20.5 inches tall, 6.1 inches wide, on a 12-inch wheel. The sidewall is 4.3 inches and the circumference is 64.5 inches, so the tire turns 982 times per mile.
What tire sizes are equivalent to 155/70R12?
Within 3 percent of the same diameter on the same 12-inch wheel: 145/80R12 (+2.9%); on a 13-inch wheel: 155/65R13 (+1.9%), 145/70R13 (+2.2%). The percentage is the diameter change, which is also your speedometer error.