pfunk wrote:
EDIT to add GCVWR is the sum of the two
And this is precisely why passenger weight absolutely matters in tow ratings. The max trailer weight of a vehicle is a simple mathematical equation of the GCVWR minus the curb weight of the truck (plus, in some cases, a driver). When you add additional weight to the tow vehicle, that weight counts against the GCVWR, and thus reduces the amount of trailer you can tow.
Example...
An 7,850 pound truck with a 150 pound driver and a GCVWR of 20,000 pounds will have a max trailer weight of 12,000 pounds. Add 800 pounds of passengers and gear to that truck, and the truck now weighs 8,800 pounds - meaning the max trailer weight is now 11,200.