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.
For the effect on road speed and distance, see our worked speedometer example. When maintaining this set, use the rotation pattern guide to check whether tire direction or different front and rear sizes restrict rotation.
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%). These are nominal diameter comparisons, not approved replacements. Check vehicle-approved sizes, rim width, load and speed ratings, and clearance before changing size.