How to Covert Metric Tire Size to Inches

Though cars are a luxury enjoyed worldwide, the standards used to define and build them vary internationally. In America especially, knowing what kind of tires will work on a foreign vehicle can be tricky business if you're not familiar on how to convert metric units into standard American ones.

1. Learn the Metric tire code. Metric tire-code, which defines the dimensions of the tire, comes with three specific 'callouts,' written ###Wx###Ar##. W refers to tire width at the widest point of the tire. A refers to the 'aspect ratio', which is the height of the tire's sidewall as a proportion of the tire's width. Finally, R refers to the diameter of the tire's rims, and is called out in either inches or millimeters. Thus, a tire with the Metric code 300/82/r14 (Or r 35.) references a tire with a width of 300 millimeters, side walls that are 82% as tall as the tire is wide, and which is meant to fit 14 inch/35mm rims.


2. Learn the American standard. American standards follow a similar pattern: ###Dx###Wr##. D represents the tire's total diameter, W is tread width, and R is rim size, as above.


3. Convert metric units into American standard ones by dividing all millimeter measurements by 25.4 to yield inches. In the above example, 300mm becomes 11.8 inches.


4. Determine the numeric height of the sidewall. Multiply your newfound width by the aspect ratio and divide by one hundred, (11.8x82 /100) = 9.67. This is the space between the rims and the beginnings of the tread, in inches.


5. Add rim diameter to your sidewall distance. This gives the approximate 'total diameter' of the tire. In our example, 14+9.67=23.67. Keep in mind that metric measurements don't have a callout for 'total diameter', so this conversion is approximate and does not include tread thickness above the sidewall. For most purposes, however, this is sufficient. Therefore, our 300x82r14 metric tire translates closely to an 11.8x23.67r14 American standard tire.