Going by those brochures, the 14FF is rated for 7500lbs if its from a C20 crewcab, or any 1-ton (SRW or DRW). Seems strange... I can't believe a 2" difference in perch width (vs. 3/4-ton) is good for another 1500lbs??? Maybe the 7500lb housings have thicker tubes? /forums/images/graemlins/dunno.gif
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im trying to GUESS what i can tow safely. i think maybe 8000-10000
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I think you might be overshooting. The 3.9L Cummins came in mostly 1-ton rated vehicles, like a GM P-30 w/10000lb GVW. If the engine itself is good for 14000lbs, lets assume that to be a GCWR (truck & trailer) with maximum axle ratio (say 4.56) and normal-sized tires.
That brochures page has a few GCWR charts. After you figure out your effective axle ratio with normal tires, you're probably down to 10-12000 lb GCWR. Subtract the 6500lb truck, and you're left with 3500-5500lbs trailer weight.
I've made a couple of strong assumptions though. You could try calling a GM dealer and see if they have the trailering info for the P-series vans. Those brochures indicate the 3.9 Cummins was available in '91 as rpo code 'LST'. Also, the 'JB8' code indicates a 10,000lb GVW brake system... the same as what the D60 and 1-ton 14ff have.