modog, welcome to the sight. ignore the big wheels need bigger breaks theory, i have already disprooved that. anything that will fit on a stock d100 you already have sufficient brakes for, as long as your breaking system is right, faulty or worn out parts dont count. 31x10.5 tires on 15x8 inch wheels fit on all four corners of a d100, i ran them for 2 years with absolutly no rubbing on a stock truck. the backspace is the issue. anything more then 3 1/2inches of backspace will rub the steering arm. but, you also need to make sure your truck has the proper alignment, the king pins are in good shape, as well as the tie rods. any of these parts can effect a wheel that would normally not rub. run the 20's, it would be sweet to see. keep your front tires under a 31inch totall diameter, and narrower then 10.5 inches wide and youll do ok with the right backspacing on the wheels. as far as out back goes, you can get away with a 33inch diameter and up to 11 1/2 inches wide. this is posted from road experience, i too had 235's up front that rubbed, swapped wheels and back spacing, and the 31's fit perfect, but a 32 rubbs the front lip of the fender. comes in handy having a buddy that works for a tire shop, we spent 3 hours and a 12pk of budwieser to see what fit and what didnt after hours one night!
by the way, this was on a 66 d100 long wide bed, no mods to suspension at all, and running manual drum brakes on all four corners.
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