...I can't agree that GCWR is based on reliability over power because I truly don't know what criteria ANY of the manfs use, much less all of them... Seems to me that whatever they use, if something changes enough that it can become a weak link, then the rating is likely to need adjustment.
Certainly, Ford generally rates the same vehicle with a larger powertrain as having more towing capability.
Yes, because the larger powertrain can do more work (haul more) before reaching its limits of reliable operation. If Ford was dumb enough to sell the Powerstroke V8 diesel and 5.4 L Triton (gas) engines with the same radiator and other coolers, same transmission, etc., then the diesel would not have a much higher (and perhaps even lower) GCWR than the gas engine.
A 5.4L Mustang has much more power - from a variant of the same basic engine - as a 5.4L F-150 pickup truck. The truck is limited in GCWR by the engine (not the chassis or structure); we know this because trucks in the same line with different engines have different GCWR ratings. For the few minutes that it would last, the 'Stang would haul a four-ton trailer with greater performance than the truck, because that's what power is all about. The "few minutes" is what reliability is all about. And that's why horsepower is not correlated to tow ratings, but engine size is.
Pete's right that we don't know all of the factors which determine the GCWR (or any other limit), which is why we can't change one component and safely declare that we have increased the rating. The pattern
is quite apparent in most vehicle specification listings, which help guide the search for appropriate configurations, but we need to be aware that ratings will be based on factors - like the altitude effect - which we hadn't thought of.
I agree with the "weakest link" assessment... and "weakest
" in this case applies in the sense of least strong, as in most easily broken, rather than least powerful. I sincerely do not believe that the "weakest link" is ever power in a non-commercial vehicle, since big rigs tow with only ten horsepower per ton of GCW, and that isn't very much compared to any of our tugs.