No precise answer to to the tow/haul rating of the springs. We don't have an official rating that I'm aware of, so I'll have to ask what the others think from their experiences and customer feedback and get back to you on that.
Basically though, I don't think they're designed with specifically more or less towing capability than the stock ones. More leaves does not necessarily mean more capacity. It "can" of course, but not always.
The main criteria are usually better articulation without losing decent and safe street handling characteristics, while maintaining a reasonable ability to carry your gear to the trails.
You're actually quite likely to sit a bit high in the rear at first too, while the springs settle in, but that doesn't mean they won't drop down some when you hook up the trailer either.
And remember too, an EB's "hauling" capacity (weight carrying) is measured in the barely hundreds of pounds. From a low of maybe 250lbs to a high of perhaps 750 lbs max I would bet. I'm basing that on expected and actual curb weights of from 3900 to about 4200 lbs with stock-ish to mildy modified rigs. With GVWR ratings of what, 4400 to 4900 lbs? I forget what ratings they had, but that sounds about right.
That would include not only spring rates, but wheel bearing and axle size, and especially brake size.
So along with your approx. 350 lb tongue weight (based on a properly balanced standard 3500lb trailer) and a couple of people and stuff in the back, you're pretty close to, or over the maximum factory haul ratings right there. And I doubt that an EB has anywhere near a 3500 lb tow rating.
To that end, as you probably know, NO spring or helper will increase the legal tow capacity of any vehicle.
I don't remember what an EB's actual rated towing capacity is. Most likely between 1500 and 2500 lbs MAXIMUM if even that. I can't remember ever having seen anything in print.
Maybe this is a good time to bring that subject back up actually. We know plenty of us use our rigs to tow overly heavy loads and usually get away with it, but does anyone have a brochure or paperwork showing an actual rated and stated towing capacity from Ford?
I'd sure be interested in seeing what they said about that.
In the meantime, I'll get some better info (if it exists) and let you know.
Paul
PS: Oh, and since you're talking about towing, what condition are your rear wheel bearings in? If they've got over 75k miles on them, or you don't know their age, put new ones in BEFORE you tow.
