New here, spent a long time looking through various forum posts and using the search bar, not sure if its limited access from not having an account or otherwise.

Anywho; I have a ECLB 460 F250 ZF5. Almost immediately after buying it I got a very good deal on 05 350 axles.
Aside from the obvious reasons for swapping, these axles have 4.30 diffs. Truck came with 3.55s and towing was meh.
That being said, upgrading to the bigger axles, and deeper gear sets, with the wide ratio ZF and 1 ton springs; what is a reasonable (safe) 5th wheel weight to hookup and run?
Under 36ft long is the limit I personally want to pull with an SRW. But the weight has me mixed up. I have a manual (and have read through it many times), as far as I can tell there is no distinguishing between DRW/SRW and 2x4/4x4, just engine/trans/diff (Tho you can assume at 26k GVWR that would refer to an F-super duty)

I will not be towing long distance, but the towing I have in front of me is roughly 3000ft of elevation gain over 45 +/- 10 miles
grades up to 8%

My gut tells me to stay under 10k, but based off the manual and math, the truck could do 10k +/- 1k
Thoughts are welcome and invited, hopefully I have presented a clear case
Thanks!

 

Your truck has a VERY LOW tow rating compared to a modern truck, in fact it will be quite a less than a modern F150. I don't have an owners manual anymore so we will have to wait for somebofy that does, that is where the ratings were published.

 

According to the owners manual my truck with 4.10 and ZF5 is good for up to 16k GCWR, my truck weighs in at 6440lb with me in it and half of one tank of gas.

 

That seems more like the auto trans tow ratings form what I remember but I could be wrong. Lets say it's correct, the limiting factors with these old trucks were the clutch and brakes, if you swap in a rear axle with disc brakes then you improve that situation but you still don't have a lot a HP under the hood so the truck will seem to struggle with bigger loads, so best to not over do it with the trailer size and weight.

 

My 1996 ECLB 7.3/ZF dually is rated around 10000lb trailer. The brakes are small on these. Sure you can pull more, DOT stops you...on your own.

4.30:1 would be about 1000rpm more than your current cruise RPM, all else equal. I would do it if tow pig. Direct and OD same.

 

Here's the thing. Yeah the gears will help your towing. But guess who doesn't care that you upgraded gears and brakes? D O.T. they will still nail you to the wall because your door tag is the LEGAL limit you can have.
Hell I flat towed my uncles 2001 7.3 super duty and LOADED dump trailer that together were probably 16,000 lbs. Did the 5.0 5 speed 3.55 geared f150 like it. No. Would it do it ? Yeah the tandem axle trailer did all the breaking and it was flat road for the 7 miles and we never broke 25 mph.
I'm pretty sure DOT would have just shot everyone involved with that mess instead of writing tickets had we been caught. But just because the truck would do it doesn't mean I'll hitch to 6000 lbs and try it either.

 

I agree with all the posts up above. I've hauled some sketchy heavy loads myself and in my 94 F150 it's a chore to keep it between the lines on the road as it is without a trailer. Unless that truck is in top notch condition I would stay with a smaller camper and under 10k lbs. Make sure your steering has little to no play and don't push it to it's breaking point. I would be worried a u joint would detonate itself going up hills with tons of weight on them and I've seen it happen a few times with these old rigs.

 

16K GCWR matches the 460/manual trans/4.10 rating from my '97 F-250HD manual. Dropping to 3.55 gears drops it to 12K GCWR. FWIW, 460/auto/4.10 goes up to 18.5K, with 3.55 gears dropping that to 15K.