Here are measurements I took from BF Goodrich T/A KD 315/35r17. As you can see, these tires measure 12.75" or 12-3/4" in width. These measurements were taken with the tire mounted on a stock 11" wheel and properly inflated.
Note: in order to provide the most accurate measurement, I used a straight board and placed it over the tire and simply measured the distance from the ground to the board. Don't be concerned with the tape measure looking crooked, the photo is what is crooked.
