After having shopped for towing equipment for my new truck, I agree that one must pay attention to the ratings of the ball and the ball-mount (drawbar) and if a rating of 10,000 lbs or 12,000 lbs is desired, you may have to go to a more specialty store. It was suprising and bothered me a little that the ball size wasn't really a function of the overall capacity of the towing setup.
To determine the towing capacity, you really need to look at the Gross Combined Weight Rating and subtract the "Curb Weight". The base GCWR typically varies by the base model and engine size. To get the advertised "available" towing number, you typically have to purchase the least-optioned truck (except you typiclaly need the towing package) with the largest engine and shortest wheelbase.
If you want to get nit-picky down to the nearest pound, the manufacturers typically publish a very detailed table of how much weight each option or feature adds. Options like 4x4 or a crew cab typically cut into the rated payload and towing capacity significantly. Leather seating, up-sized wheels, and other "trim" items each add a few pounds.
Or, take the truck and weigh it with you and whatever other passengers, cargo, and payload you intend to carry while you tow. Subtracting this weight from the GCWR for that model gives your remaining towing capacity.
Unless you plan to weigh your loaded trailer and loaded tow vehicle, I think you should leave a little margin for error in estimating. For example, towing my boat. I know the boat weighed almost exactly 4000 lbs. I know the trailer weighs abut 1000 lbs. My poor little Dodge Dakota was only rated to pull 5000 lbs. Theoretically, (dry and empty), I may have been right at its rated capacity. In practice (with fuel and stuff), I bet I was over by quite a bit.
Add a couple hundred pounds of fuel in the boat (I always filled it up whenever it was on the trailer due to convenience). Add another unknown amount of water in the bilge and soaked into the carpeting and seating. Add another hundred pounds of "stuff" that just tends to always be in the boat like the life vests, extra towels, skis, tow rope, wakeboard, small toolbox, fire extinguisher, anchor, extra anchor, paddles, fenders, rope, and the stereo. Add all the stuff you pack for the day into the boat like the cooler, food, chairs for the beach, toys for the kids, and that large inflatable pull-behind.
Now add all the stuff riding in the truck - payload and passengers in the truck count against your GCWR as well. Me, my wife, kids, and dog weigh in at maybe 450 lbs. Add some suitcases, more food, lawn chairs, sleeping bags, and fishing tackle for another couple of hundred pounds.
So on a typical weekend of towing my "5000 lb" boat and trailer, I was actually hauling a lot closer to 6000 lbs of stuff in my combined towing and payload weight.
Thats why I just upgraded to a bigger truck (Chevy 2500) that really is rated to pull around 10,000 lbs. Do I care if its exactly 10,700 or 9,500? Not really because I am leaving myself a bit of margin and only plan to pull 6000 to 7500 lbs.
If I really needed to tow close to 10,000 lbs on a regular basis, I would move up to something with a 12,000 lb rating (2500 HD or 3500HD in the Chevy lineup).
- Rick