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I hope i'm doing this right, but in my head this is what you do:
(Don't know what your current wheel width/backspace is, the following is an example).
1. Current wheel is 8" wide w/ 5" backspace. Subtract the two and that means that the outside wheel edge is 3" from the hub face plate (forgot what it's called).
2. New wheel is 10" wide w/ 6.09" backspace. Subtract the same way to show that the outside wheel edge is now 3.91" from the plate.
3. So now the wheel extends almost 1 inch wider than the one before did (9/10" or .91").
4. If the new tire is wider, the surface of the tire should be 1/2 that difference wider as well. Let's use as an example going from a 10.5" wide tire to a 12.5" wide tire. It is 2" more but is 1" more in each direction from centerline, so it will add 1" to how far the surface of the tire extends out, and 1" farther into the fender well. So, the outside wheel edge extends .91" more than before, and the tire surface extends 1.91" more than before. And 2" is a pretty big difference, I think.
I think i am right, but if i'm wrong someone feel free to correct me.