Unless they botched the math, GVWR should be the car with full fluids plus payload/passengers.
Seems to me that most people aren't carrying much payload when they drive. If so, being overweight probably doesn't reach the car's GVWR and present a hazard.
On the other hand, I have seen estimates of the enormous number of gas gallons it costs to haul around overweight Americans. Here's a link:
Erdrick 
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