Roy,
Larger trucks do and will have GCWR ratings per say. Not like pickups in that their is a single rating.
A given chassis drivetrain may have multiple ratings depending upon the end users needs. If used as a local delivery, gcw might be lower than an otr setup. Because local needs and wants say a 20-25% gradability, only 55 mph speeds on freeway. Meanwhile the otr setup wants 60mph, and only a max 15% gradeabilty.
It ultimately does get down to axle gears, tire diam, HP, torque, transmission gears ratios, number of gears....
Some rigs line my Navistar, reality is it's never supposed to be going down the road beyond manufactures gvw if 18200. Alot of us push this motor local deliver to 26000. It does not do real well beyond that. Even if it's level.
Marty