GarageCalc How to read the 12-inch chart
Every size here has a 12-inch bead diameter, but it also needs a suitable wheel width and vehicle approval: the diameter spread is 1.2 in between the shortest and the tallest. 0 of the 3 sizes have an aspect ratio of 55 or lower, the low-profile end used on cars, while 3 sit at 65 or higher, the tall-sidewall sizes used on trucks, SUVs and vans.
To assess another size, first check the vehicle-approved options and tire manufacturer's permitted wheel widths. Similar overall diameter alone does not establish fitment. The comparison calculator shows the speedometer error for any pair, and the offset calculator covers clearance if the wheel changes too.
Comparing wheel diameters? Our 17 vs 18-inch worked example shows why the complete tire size matters. The speedometer guide explains the effect of a diameter change.
Frequently asked questions
What is the most common 12-inch tire size?
Width 155 mm appears most often in the 12-inch catalogue, across 2 aspect ratios. Which one your car takes depends on the placard in the door jamb; the chart lists all 3 sizes so you can read the diameter of each.
How tall are 12-inch tires?
Anywhere from 20.5 in (155/70R12) to 21.8 in (155/80R12) in overall diameter. The wheel is always 305 mm; the sidewall, which is width times aspect ratio, decides the rest.
Can I put a different 12-inch tire size on the same wheel?
Matching wheel diameter alone does not confirm fitment. Use vehicle-approved sizes and check the tire manufacturer's rim-width range, load and speed ratings, and clearances. The 3 percent diameter filter on size pages is only a comparison aid.