I suppose you are right on the calipers not kicking it out. I oly had one instance where they loaded up with mud and that was after a lot of rain andi went a little farther off road than usual. I am surprised at the tire clearance they have. Originally I went with 38's.
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80 Mercian Olympic, 92 DB Overdrive, '07 Rivendell AHH, '16 Clockwork All-Rounder