tire rotation on AWD

On a 215/60/16 tire on my Forester the maximum difference in circumference should be 1/4 inch or 0.250. A 215/60/16 tire has a listed diameter of 26.0 inches. If you put a decrease of 0.250 inches into the equation then the maximum difference in tread depth is 0.040 inch which is close to 1/32 or 0.03125 inch. I rotate when my front to rear tread depth difference is ~ 1/32 inch. Sometimes that occurs at

You also need to take into account tread wear difference at sides and center of tread, it's not all about keeping the same diameter on all 4 wheels it's also equal wear across the tread. Balance should not be "necessary" every rotation, perhaps checking it every other rotation depending on the driving conditions of course.

If you care for a more accurate way of checking tire diameter, here's what I did. Put a chalk mark on each tire's side in the center of the contact patch with the ground. Roll forward in a straight line (parking lot on sunday or small residential street) for 40 revolutions of the tires. I measured the approx diameter of the tire X 40 and put a mark on the ground. You could also have a helper counting the revs. If the circumference diff front to back is 1/4 inch or more, you would find that your front and back marks are 10 inches or more apart (radial measurement. What I like about this method is that it takes into account all variables : center and side tread depht and air pressure in the tire (have your tires at normal pressure).