GarageCalc What 145/80R12 means
145 is the section width in millimeters (5.7 in). 80 is the aspect ratio: the sidewall is 80 percent of the width, so 145 × 0.80 = 116 mm (4.6 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 × 116 = 537 mm, or 21.1 in.
A full marking adds a load index and speed rating after the size, for example 145/80R12 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 145/80R12
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 145/70R13 at -0.7% diameter with a 102 mm sidewall instead of 116 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 145/80R12 tire?
21.1 inches (537 mm) in overall diameter: the 12-inch wheel (305 mm) plus two 116 mm sidewalls. Actual dimensions depend on the tire model, tread depth, pressure and load; check the tire manufacturer specifications.
What is 145/80R12 in inches?
21.1 x 5.7 R12: 21.1 inches tall, 5.7 inches wide, on a 12-inch wheel. The sidewall is 4.6 inches and the circumference is 66.4 inches, so the tire turns 954 times per mile.
What tire sizes are equivalent to 145/80R12?
Within 3 percent of the same diameter on the same 12-inch wheel: 155/70R12 (-2.8%), 155/80R12 (+3.0%); on a 13-inch wheel: 145/70R13 (-0.7%), 155/65R13 (-1.0%). The percentage is the diameter change, which is also your speedometer error.
Can I use 155/70R12 instead of 145/80R12?
Nominal size comparison only: 155/70R12 is 2.8% shorter (-15 mm) and 10 mm wider, and at a true 60 mph the speedometer would read 61.7 mph. This does not confirm fitment. Check approved rim widths, load and speed ratings, and vehicle clearances. The nominal extra width adds about 0.20 in of clearance on each side of the tire and a rim width that suits 155 mm.