omgosh are you serious only 4mpg pulling 15.5K that's horrific!!!!
I pulled 10K with a 351 Windsor HO with a 4bbl and a C6 trannie in my 85 F150 - yes I know it was way too much weight but I did it all the time.
I would get 6.5 to 7.5 with it running 65 to 70 mph.
my 6.0 p/s gets 12 to 13 pulling that kind of weight if I roll through the hills vs hammering on it.
If I hammer it 9 to 10
Engine Build Oct 2012, Truck Build Date of January 2003 One Owner, 6.0/Torque Shift 5, F250 XLT CC FX4, Camper Package, 6 Disk, ( now JVC/Bluetooth - Sub/Amps ) Dual Power Seats, Privacy Glass, No Tuner, Air Raid Intake, 4" Exhaust/5" SS Tip, 299K Miles currently, @ 160K Bulletproof EGR Cooler, ARP Studs, New Oil Cooler, New HPOP, New Head Gaskets, New EGR Valve. Rusty fender wells and lower inside of doors. Showing her age. Original 03 Turbo, 8 Ford Reman injectors @220K, had problems with 3 so far, one cracked tip. DC Power 190 alternator, DC Power Wiring upgrade kit, FICM.Com lifetime Economy Tune @220K miles.