Roy,
Scooby/noteven probably said it better than me. In that GCVWR is based, speed etc on the end users needs. Shiner hinted when he mentioned truck used in a mine get speed with heavier duty components, yet on the road, they can't haul that amount due to road bed design limits.
This mine rigs lig garbage trucks might also have a gradubikity of 100% ie 45 degrees. A typical otr rig will be stalled out on a 15-20% grade. Pickups are only required to be a min if 12%, doesn't do me a lot of good sitting her in front of my sister's with a full load on an 18% grade. Or a past client who's driveway was 30%. I can't go up those grades unless I back track on pickup GCWR ratings, deduct for hight grades
Pkud HP ratings on a truck can be deducted 30% if a given truck has a full aero PKG, plus 30% is needed if hauling a trailer like a log truck, chicken hauling rig, as wind resistance is way higher then they will use actual frontal area, not just 80 sq ft, with no deduction if your hauling a 120 sq ft trailer behind your pickup.
Reality, I'm hitting the tip of the iceberg in how much different pickups vs heavy duty rigs are speed to do there jobs. I've stalled out multitudes if pickups on local steep grades, u set rated GCWR. I've seen an 18 wheeler that did 60 mpg from Louisiana to Wa st, stall out on one of our fun local 20% grades. A mile from it destination...
This is why I bought the Navistar I've an equal GM it Ford. They gave me the what and how it would handle what I was doing, vs a shrug of shoulder, wave of hand, no problem!
Any way, time to go cut some wall block caps finish what I'm doing here
Marty