Tire Sizing Explained

Competition	Street
16/64-15	195/65x15
16 = tread width in cm 64 = overall diameter in cm 15 = rim diameter in inches	195 = "section width" or tire width in mm 65 = "aspect ratio" or the relative height of the sidewall in relation to the section width. i.e. 65% of the section width. 15 = rim diameter in inches

As you can see, the competion sizing method gives you the more functional dimensions of tread width and overall diameter. On street tires the tread width is not always the same for the same section width. In fact, the measured section width will also vary depending on what size rim the tire is mounted on making the street tire sizing method even less useful.

The overall diameter of the tire is not specified in the street tire method but can be calcuated with a little math. In the example above the height of the sidewall can be determined by multiplying the section width by the aspect ratio expressed as a percentage. Thus 195x0.65=127. As there are two sidewalls, top and bottom, we multiply by 2 giving 254. Now add the rim diameter converted to mm: (15x25.4=381) 381+254=635. In this case the street tire and competition tire are almost identical and are, in fact, considered equivalent.