Astoria Dave is right when he says there is a lot to consider when it comes to tow capacity - and a lot that dealers (and even some "experts") aren't familiar with. I found that as a consumer looking for a tow vehicle (and worse, combining it with a camper), I had to learn how to interpret the information on my own.
For example, on tow ratings. They may give a firm figure, such as "5,000#" BUT, that may or may not mean it's up to the task. Essentially it's like a ground tackle system, in that you have to figure out the weakest link and then use that for sizing. I've found the hidden "stopper" to often be the rear axle rating, especially if the vehicle has a long overhang (because then you have to figure in lever arm, etc. between the hitch weight and the axle). But it could be another dimension. Main thing is to understand the numbers, know what weight you'll be towing (and what weight you will have in the tow vehicle), and do your own math vs. trusting a sales person or even a brochure. Plus, as Dave says, do some research on transmission, etc.
I'll give you my vehicle for an example on the weight ratings:
3/4 ton van
GCWR = 13,000# (gross combined, i.e. rig plus trailer)
GVWR = 8,600# (gross vehicle, i.e. tow vehicle max weight)
Published tow rating = 7,500#
Okay, that looks good, right? 7,500# is a lot.
BUT.... like the ground tackle system, there are hidden "stoppers" that you can figure out in advance to see if it's REALLY going to be a good tow vehicle.
Right away, without even weighing anything, you can see that if you had the van loaded to its max, you would only have 4,600# left over for towing (which is not to say you would have it loaded to the max, but it's something you can look at right in the specs).
Additional things to know:
GAWR - front = 3700# (maximum weight allowed on front axle)
GAWR - rear = 5120# (maximum weight allowed on rear axle)
Distance from rear axle to hitch ball = ~52"
Next is to weigh the van alone (but loaded for travel, including driver and passenger, full gear, etc.) This weigh was at a super-loaded state --- but that needs to be possible for my travel style:
Front axle of van alone weighed at scale = 3480#
Rear axle of van alone weighed at scale = 4380#
(note that towing shifts a bit of weight off the front as well as adding to the rear)
Okay, so already we have around 7800#. If you subtract that from the 13,000# gross combined rating, it leaves around 5200#. I'd have to strip the vehicle to ever have the "real" 7,500# available to me. And, check out the rear axle.... there's only around 750# left there. The tongue weight of my trailer is around 330#, BUT, when I figure in the 52" overhang, I get a effective addition to the rear axle of 460#, bringing me up to....
Van plus trailer, weighed at scale:
3340# front axle
4840# rear axle
So, I'm not right up to the max, but I'm getting close. And this is towing a ~4,500# load --- not even CLOSE to the supposed 7,500# tow rating. Which isn't to say that's a lie, because if the van were an empty cargo van, and I was towing something "four down" so there was no tongue weight.... then maybe. But realistically, very few vans/loads will be like that. And yet, looking at the Ford literature, you see that "7,500#" tow rating right off the bat. Not to pick on Ford; I've found it similarly valuable to "do the math" on other potential rigs. Some were eliminated right from the get-go.
Normally, my loads are slightly less than listed above, because when I did that weigh I had a buddy along - a bigger guy and extra gear - but then I'm glad to have a vehicle where I can do that if I want to.
Another note is to check tires. Sometimes it might be good to go up a load range from the originals. The actual hitch (i.e. metal receiver part) is another thing to check - but that can often be beefed up if the other vehicle specs support it.
For the transmission, etc.: I run a Scan Gauge II, which is a nifty little gadget you can plug into an OBDII equipped vehicle (around 199x and later). It reads out transmission fluid temperature, water temperature and a bunch of other handy stuff, so you don't get any sudden surprises. I wouldn't be without it now - really nice to keep an eye on things (with real numbers) as you go along.
You know, in some ways maybe it's good that I bought a used boat. On the one hand, I got a mixed bag - that is, some things I would have chosen just the way they are, and others that are fine but maybe not the exact thing I would have picked (and some surprises when things I wasn't that fond of turned out to be better for me than what I would have chosen). But then too, I didn't have to figure out exactly what I wanted for each detail. That's fun, but also a bit daunting --- are you on information overload yet?
