All my time behind the wheel of my 92 gmc c3500 cc dually with the 454 and I never got better than 10mpg and that was not towing anything!
So I decided to go diesel, 1995 cummins 12v p pumped turbo diesel engine plus a nv4500 5spd transmission out of same truck.
Almost have it running and will report back on what I get for mpg but should be in the 13-16mpg towing and 18-25mpg unloaded.
I still plan to get a doug nash underdrive orverdrive box to further increase mileage including regearing truck down from 4.10 gears to 3.54 locker.
Oh did I mentioned I plan to run waste oil to burn also so basically I will run my fuel for almost free when I can get it.
Most of the info I got was from various forums like 4btswaps, cumminsforum, diesel bombers, duramax forum.
Speaking of duramax swap yeah u can do that too but its super expensive and does not make senseto make a cheap and reliable tow rig like minds, I have less than 5k into mines, down the road when things get cheaper I want to go with the new 6.7 cummins to make it nicer and quieter