8" if you are using ICON/Deaver springs as they move the front axle forward 2"
Any other brand may or may not move the axle and then you will be looking at 10"
I have the ICON's on my Excursion and the tires fit nicely. For a little extra insurance I also cut the bottom of the wheel liner at the rear and tucked them back along with moving the bumper out 1".
I would rather be safe than have to replaced a smashed fender.
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