the REAL towing capacity of my GMC Sierra plz help?

[superman]

Ok, so I have a 2001 GMC sierra extended cab, short bed, 4.8 4WD. Now, according to the manual, I think it states that I could tow 7000 lbs. However, I want to get a dump trailer and sometimes, when the dump is full, it might weigh 12000 pounds. Now, does anyone know if i could (every now and then) haul 10-12000 lbs. about 20 miles to dump it? I mean, I could go slow. Is this something that would be really bad, or illegal? Most of the time i could probably keep the load at 8000 or so.

Any help would be appreciated.

thanks
I assume i would use the tow haul button and take it really easy, you know

 

[chevypride0310]

well not sure if it actually could but even if it could it will blow the transmission, even if you get a transmission cooler it still way to much than the transmission is designed for, thats almost 2 times the weight it can tow, that kind of weight you need at least a 3/4 ton truck, also 20 miles might not seem like much but its a long way for pulling a trailer twice the weight it can tow safely, plus what if you get to a hill? even at the 8,000 it will still blow the transmission just won't take as long, usually i subtract about 500 lbs from the capacity and thats the safe weight, because the rated capacity i think is for straight level ground, not hills, so your maximum weight is probably 6,500

back when i was in high school a friend used to tow trailers with his truck that was heavier than what the truck was rated for, it could tow them BUT he had to get the transmission rebuilt more than once in a short period of time