Under load, the tire flattens on the ground. For this to happen, the tire must abrade (slip) on the ground. Consequently, the loaded circumference is less than the unloaded one. The rolling radius is therefore less than the half the diameter of the unloaded tire. As the wheel rotates, the distance from the center of the wheel to the ground remains the same as new rubber is positioned under the center of the wheel. Hence, it's the loaded radius that must be used in calculating the circumference. This reduction in the rolling radius is what I've never been able to find published probably because for the same nominal radii of tires, the width of those various tires is variable, and this determines how much the tire will be deformed (non circular) under load.