I did some heavy towing with my 91 d250 and I was amazed. My trailer is an 20 ft bumper mount equip trailer, 14,000lb axles, and the darn axles are closer to the rear so it puts more tongue weight on the truck.

a 1500 1/2 ton truck will sag with just my trailer on, nothing else.

My f-350, dually with overloads sagged a good 2-3" with just the trailer on...

and now my 91 d250 with what looks like reg LD springs, moves about 2-3"...the same as my old dually did. I was floored. I guess alot of it has to do with the make up of the spring steel its self and not just the plys. I plan to add a few plys just because...

 

I was pretty surprised when I hooked onto my 5er and she didn't squat at all. Granted, it's only a 1000 pound pin weight. What I've found now is that the springs are so saggy that they actually can't go any further...doht! I put my Hi-Lift jack in the hitch reciever and cranked it up till the springs had just a shade of positive arch---this was about 6''!!, but the truck looked level. I've decided to just replace the springs when $$ allows.

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mine haven't sagged a bit, and I can't get them to. The front comes up with a heavy load but the back might go down a couple of inches at the most.

Rides like it to. I was thinking of removing one or two from the rear pack.

 

Mine likes to squat. It sags pretty heavily 4-6" with my 7'/14' enclosed. It scared me to death pulling loaded at 10k. I think some sway bars and stiffer springs would help though. The wandering all over the road in the rain is frightening.