Ex) 235/65R17 108V : 65 <- aspect ratio

The aspect ratio is the ratio of the height to the width of the tire.

In the past the ratio was usually 100, meaning the height and width were the same. Recently, the trend has moved towards a lower and lower aspect ratio -70, 60, 50, 40, etc.

For many of today’s tires the aspect ratio is referred to as the "series." If the aspect ratio is 65, then it is called a 65 Series tire.