I'm going to disagree, though I will test the gravity and get back to this thread. Regarding the flush. I have a steep driveway and habitually drain the 80 with the front downhill, heat on, lower hose off, block drain plug out, radiator drain open, upper hose off. Last time I drained, I also started the engine for just long enough for it to catch and shutdown, which yielded perhaps a pint.
Unless there's a serious blockage somewhere holding a lot of fluid, then I cannot explain it except to assume Toyota mislabeled the capacity. I've owned this truck since new and the coolant has been changed ahead of schedule over those 10 years by me personally each time. Ditto the thermostat, which has a bypass valve and I've never seen any of the 3 I've installed come out with the bypass clogged. If anyone has been able to get the full listed capacity into their truck, I'd like to hear about it.
I can explain it no other way, but I've never checked the gravity and will do it this week. As a final comment, this truck has frequently pulled a 6000lb trailer over mountain passes at high temps with nary an issue.
Doug