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
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. 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 145/80R12 tire?
21.1 inches (537 mm) in overall diameter: the 12-inch wheel (305 mm) plus two 116 mm sidewalls. Real tires land within about 1 to 2 percent of that depending on brand, tread depth and pressure.
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?
Dimensionally yes: 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. The extra width needs about 0.20 in of clearance on each side of the tire and a rim width that suits 155 mm.