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From the GM website, a 2500HD truck with DMax/Allison is rated to tow a 15,500 lb. trailer (remember, single wheel, 3/4 ton truck, not even a Dooley). You are right about two average pickup trucks weighing 6000 lbs./each, but we all have seen 4 wheel drive 1/2 ton and 3/4 ton trucks shown to weigh much less. A gooseneck trailer long enough to to fit these two trucks will not weigh 7000 lbs. as you described, but more like 4000 lbs. (or less).
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Yes, a 3/4 ton may be rated to pull 15,000 lbs, but if you read into it further, it's really not, and here's why.
The GVWR of that truck is likely to be 9,200 lbs. Loaded with fuel, the hitch it self, passengers tools, etc., especially if that truck is a crew cab, it is going to weigh AT LEAST 7,000 lbs. 9,200 - 7,000 = 2,200 lbs left over for pin weight.
My 33' 5er has an EMPTY pin weight of 1,800 lbs. The reality of it is that no matter what you do, you're going to bust the GVWR before you get to that 15,000 lbs. In fact, you'd be lucky to be pulling a 10,000 lb trailer with 2,200 lbs of pin weight or less.
That is saying that the truck only weighs 7,000. If it is a crewcab 4wd, I'd bet it will weigh even more. More like 7,500 or so. That is going to give you even less of the GVWR available for pin weight, and if that truck weighed 7,500 lbs and had my 33' 5er hitched to it, it would be overloaded pulling an empty trailer that only weighs 8,500 lbs.
So, you never busted the, "gross trailer weight rating" that the factory gives you, but you busted the GVWR. Moral of the story? You're nowhere near the rating of the truck's total pulling ability but you're overloaded.
We have also not considered the rear axle's weight rating or the rating of the tires in the rear of the vehicle which are likely to allow even less weight available to pull safely within the limits set forth by their manufacturer which are assuming that you're at the max pressure as well.
4,000 lbs or less is complete and utter nonsense. If my 20' trailer weighs 2,500 lbs, you'd be a fool to believe that a 30+ footer with axles that have dual tires and that long gooseneck hitch hits the scales at less than 5,000 lbs. No way you're going to get a steel 30+ foot gooseneck rated at 20,000 lb GVWR that is going to be 4,000 lbs.
Engineers design safety factors in to vehicles for a reason. This is not intended to be broken. Sure, the vehicle isn't going to explode if it's one pound over weight, but that safety factor is not there so joe blow can overload his truck sucessfully. It is there to ensure that in ANY condition, your truck and trailer combination will be safe at its full rated load.
I will not argue as to whether the truck will do it or not. That is unquestionable. I have no doubts that I could put 30,000 lbs or more behind my C30 SRW and pull at highway speed. The truck will do it, but that doesn't make it right, safe, or legal.
So, since you wanted facts only, I'll recap:
1. You will be hard pressed to load a single wheel truck to its maxiumum towing capacity without overloading it somewhere else, such as rear tires and rear axle weight rating. Realistically this huge gooseneck you're talking about with a 5,000+ lb truck right above the bed or slightly behind is going to have at least a 4,000 lb pin weight which no matter what you do with a single wheel truck, it's going to be overloaded.
"Know it all" or not, it seems that some of you guys who understand some very complicated things about four wheeling haven't read far enough into tow ratings to realize that the weight rating given by the manufacturer, or GCWR, isn't the end all to this discussion. There are a number of factors that play into whether a truck is overloaded or not that you're all missing.
Without actual numbers of a loaded vehicle and how much weight is on each axle it's hard to call whether a vehicle is overloaded or not. A single wheel truck with a trailer 30' or longer, especially a gooseneck or 5th wheel, is most likely overloaded though. A 35' trailer behind a single wheel truck is most definitely overloaded.
This is something that we really need to have a discussion about on this board, anyway. It's long overdue.