GCVW is not the GVW of the pulling vehicle plus the trailer rating. It is calculated diffently and takes into consideration the power of the tractor/truck, it's driveline components including brakes, etc. So you could pull a very heavily built trailer but it would not increase the gross COMBINED weight rating of the pair just as a small trailer would not increase the truck rating. Each component has a specific rating that should not be exceeded but the total of all rated components should not exceed the GCVW. What it comes down to is the sum total of the "least" rated components that determined the GVW. So as Steve Ley pointed out putting larger heavier tires on the original smaller wheels/rims doesn't really change a thing as far as calculating a GVWR or GCWR. Larger capacity wheels and tires together still would not increase axle capacity if they exceeded the axles own rating.
Interestingly enough, it is the Final Stage Manufacturer who must declare what the GVWR and GCWR are on a new vehicle. This final stage manufacturer is your local body installer/upfitter. He must also calculate the CG for each unit and declare tire/wheel size along with air pressure. The motor companies have put this monkey on the backs of small business even though they built the original vehicle. This also means accepting every concievable safety standard as declared on the Incomplete Vehicle Document. This explains why one so often see's tag axles added to trucks that carry no capacity increase on the GVW decaration.
As far as the origination of the terms one ton, 1-1/2 ton, etc. I must side with those who claim it refers to the front axle carrying capacity. That is what I was taught by the old timers back in the 60's when I got into truck equipment business. Some of those guys were around in the 20's and 30's.
