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We use a very wide 700c tire and it is right at the edge of the clearance for standard fenders. It is a little tricky to find the point where the wheel is clearing the fender evenly put it is possible to achieve.
Use a Philips screwdriver to unscrew slightly the screw on the back fender stay holder (fender stay acorns). Then you can slide it up or down along the edge of the fender. When you have found the point where you have consistent fender clearance along the entire wheel, turn the screw to lock in place.
You can also pop the fender stay acorn off of the edge of the fender once you unscrew it. You can also pull the fender stay strut off of the fender stay acorn. Dont worry, you will not break it.