Payload is based (by the mfr.) on frame strength & powertrain capacity. A dually has higher curb weight, so it technically has lower payload capacity than an otherwise equally equipped SRW pickup. BUT-there is FAR higher rear GAWR rating on a dually-for instance, my F-450's rear tires are rated 2805 lbs. each when used as duals, or 11,220 total GAWR. The same tires (if I used them) on my SRW GMC are 3042 each, or 6084 lbs total. Guess where (almost) all of the weight in a pickup goes?