Just remember regardless of the tow rating, legally
you cannot exceed the CGVWR (Combined Gross Vehicle Weight Rating) - that is the truck, trailer and everything (including passengers in them). There is a misperception that adding air bags or a better suspension increases the CGVWR - it does not from a legal standpoint. Yes, those items do increase the true capability of the truck (physically) but not from a legal standpoint. If you were to have an accident, and the authorities suspect it is due to being overweight, the only thing they will check is the original OEM ratings - not anything that has been done aftermarket wise.
Example of how tow ratings are not accurate.
I have a 2006 Dodge Ram SLT Quad Cab Short bed Hemi w/3.55 gears. According to Dodge, my Ram has a tow rating of 7,750 lbs.
Let's look further:
CGVWR = 13,000 lbs (legal weight limit of truck & trailer loaded - this cannot be exceeded)
GVWR = 6,700 lbs
Weight Empty (no passengers or cargo) = 5,200 lbs
Payload Rating (GVWR - empty weight) = 1,500 lbs (now subtract the weight of any passengers - say 500 lbs) and now the "true" payload rating is 1,000 lbsActual towing rating (CGVW - GVWR) = 6,300 lbs
So how did Dodge come up with a 7,750 lb tow rating for my Ram? Simple - they used the difference between the CGVWR and the empty weight rating, not the GVWR. They are not "wrong". but as the owner you need to realize you need to factor in payload (passengers included), tongue weight and 10%-20% safety margin to get your "true" tow rating.
The following link has a good calculator for towing. It's more or less for those towing travel trailers but the concept applies regardless of what you are towing:http://changingears....weight-tt.shtml