The assumption on all three big mfgrs. is tire loading. They de rate the maximum payload to have a large ‘margin of safety’ taking the tire loading into account. The de rating can also have something to do with wheel max; spring rate and optional secondary springs; and axle strength, mostly due to bearing load and housing strength; and transmission strength. From my perspective, there is one other factor: The Corps of Lawyers who work night and day to avoid any appearance of impropriety that ‘might’ lead to litigation. Also from my perspective is how close Ford, Gm, and Mopar get to the max rating line when developing a number. Ford is the clear winner as the company claims a loading number that i call “honest but dangerously close to the line” when inventorying their trucks. GM is about in the middle, while Mopar under rates their equipment as far as loading is concerned. This contest changes constantly, year by year with one of the big 3 jumping out in front with 32K pounds of towing ability or such. Much of that has to do with the strength and durability of the transmission that year.
2020 Ford F-350 XLT FX4 4WD SRW SB SC 7.3L Godzilla Gas TorqShift 10R140 397 amps dual Alt dual batts Frnt Dana 60; Rr Dana M275 E-locker 4.30's 4580/4320/4066# payload 7243# curb wt. 11,300# GVWR 5-er prepped. 2020 Northstar Laredo SC, 12v compressor fridge, cassette, 320w Solar sub zero insulation.