Ok, just a quick calc for us 'Muricans:
Unloaded:
(180km x 1.0 mile/1.6 km) divided by (0.25tank x 34 gallon) = 13.2 MPG
Loaded:
(176km x 1.0 mile/1.6 km) divided by (0.75tank x 34 gallon) = 4.31 MPG!
You said 120 Liters, that's about 32 gallons. I used 34 gallon tank, which is what literature says, but relatively speaking the numbers are off the charts. That sounds REALLY bad, LOL.
OP mentioned '91, that's the first year of the 4L80. He additionally mentioned he tows in "drive," not "overdrive." I'm not sure what that means. I think that's on "3?"
Are there a lot of mountains there? Altitude? Have you had this truck for a while? Or just bought it? Has it always had poor fuel range while towing? Unfortunately, I don't have much experience with TBIs, much less towing. Perhaps someone has a similar set-up towing similar weight in the similar road conditions. I've read upper 8MPG towing on Vortec, just have no idea on TBIs. We really need to know road conditions.
Thank goodness you did the conversion--I was starting to get a headache trying to do the translation in my head.......
4mpg is indeed pretty low. Should be more like 10-12 on relatively flat terrain, of course less at altitude in the hills.
My "light" K2500 with NV3500 and 3.73's pulled a 5K TT and would get around 10mpg at 6k to 9k in altitude in the mountains. It was third gear and hammer down up the hill though. In 4wd on dirt USFS or BLM roads and mileage was was like 6-7 mpg.
As Derek said the TBI's do best in the 2k-2.5K rpm range. By the time the motor hits 3500 rpms the swirl port heads plumb run out of breath.
The "standard" 2500 (not the light 2500 that was really a 1500 with the 14 bolt SF) came with the 14 bolt SF, the heavy duty options would bump the rear end up to the 14 bolt FF. I've seen a lot of true 2500's with the 14 bolt SF axle.
Boiling trans fluid ain't good, and at that point has probably already hurt the trans. The only factory HD coolers I've seen in GMT400's (which are 8 x 11 instead of the standard size 5 3/4 x 11) were in the 2500"s with the HD options and the 3500's.
The reason GM advised to tow in 3/D is so the TQ will lock up and would avoid extended runs with the TQ unlocked if driving in OD. That would make the trans boil over for sure..... On flat terrain while towing with 4.10's a guy could probably run in OD and the TQ would be locked most of the time.
Need to figure out why the trans is boiling over first.