I’ve worked in the tire business for over 20 years and the thing I’ll never understand is the way our industry came to describe the sizes of our products. I understand how to do it, but explaining it to someone generally makes all but the most hardened tire veteran’s eyes glaze over. It’s a combination of inches, millimeters, and percentages that has often made me wonder what second place must have looked like if this was the winner. People are normally interested in two dimensions- the height and width. This is an attempt to describe how to approximate those dimensions based on the size listed on the tire.

Standard Passenger Sizes:
Example:  235 / 60 R 15
235
This is the section width in millimeters (to convert that to inches divide it by 25.4)
In this case it’s 9.25 inches wide
60
This is the aspect ratio. This is the most important part of the size as it relates to what people call ‘low profile’ tires. Here the section height (from the rim to the tread) is 60 percent as tall as it is wide. Since it is a percentage, you must adjust the section width accordingly when you lower this number. Keeping the same section width and just lowering the aspect ratio will only give you a shorter tire of the same width. In the example 9.25 x 60% = 5.55
15
This is the rim diameter. To get the overall diameter of a tire you have to multiply the section height by 2 and add the rim diameter (the third part of the tire size). In this case (5.55 x 2) + 15 = 26.1 inches tall.

Flotation Sizes
Example: 31/ 1050 R15
The engineers were off the day they came up with this one.
The first part is the overall diameter in inches, the second is the section width in inches and the third is the rim diameter. The width may or may not have a period (10.50 vs 1050) but they would both be 10 and half inches wide.

* Both of these will give you a general idea of size and width, consult the manufacturer’s specifications if you want exact the exact dimensions.

-Chris