What Number on Trailer Tire Sidewall Indicates the Total Height of the Tire  

Question:

Expert Reply:

When looking at a standard size tire listing like the Provider ST235/85R16 Radial Trailer Tire # TTWPRG235R16, the total diameter will not be listed outright on the sidewall, but it can be determined by using the numbers that are listed.

For example, the above Provider tire will have "ST235/85R16" printed on the tire's side wall. To determine the total diameter or height of the tire, including the wheel, we will look at the first number, 235. This represents the section width of the tire, in millimeters. This is the measurement of the tire at its thickest point. To convert centimeters to inches, we will just divide the number of centimeters by 2.54 (1 inch is about equal to 2.54 centimeters).

Doing this we get a section width of 9.25 inches.

Then moving onto the second number of the tire listing above will help us determine the height of the sidewall, from rim to tread. The second number of 85 refers to the sidewall aspect ratio or section height. The number indicates the ratio between the tire width and height. For example, 9.25 inches x 0.85 equals 7.86 inches. A 235/85R16 tire has a section height that is 85 percent of 9.25 inches.

Lastly, the final number of 16 represents the diameter, in inches, of the wheel on which the tire is mounted.

Now that we have the section width, section height and wheel size of this tire, we can determine its total diameter/height. We will multiply our section height of 7.86 inches by two, to get the amount of tire extending out on both sides of the wheel. This gives us 15.72 inches. We will then just combine this number with the wheel size of 16 inches to get a total diameter of 31.72 inches.

I have also attached some great articles on trailer tires and wheels that you may find helpful.

expert reply by: Chris R

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