Both of the above responses are excellent!
Originally Posted by John in Santa Cruz
the trucks payload is determined by the wheels, tires, brakes, wheel bearings, axles, suspension, and frame. you can upgrade some of those, but not all of them (at least not practically), and whatever weakest link remains, thats still limiting the safe GVWR.
Yes, and it is not practical for the owner to know what that weakest link might be. An interesting discussion on this subject in the FiberglassRV forum led to a specification document
from Ford which shows that they use two or even three (depending on configuration) different thicknesses of frame rail steel in the same cab and box combination of F-150, with the thickness to suit the payload rating chosen. So that guy who says "I put the heavy springs on it so my truck is now the heavy-duty version" is mistaken.