i forgot about the 73 4x4 yard truck i had.
it was an original 318 2bbl/727, 3.91 gear, 32" tires.
with that engine, the best was about 8mpg.
with that said, it had a big "basher bumper" on the front made of 3" pipe and 1/8" plate.
the stock bed was made of 1/8" plate as well, had a 2" oak plank floor, and a 3" pipe roll bar.
total weight was about 7000lb empty.
put a 440 in it out of a 1970 chrysler wagon that was in the yard, added an rv torque type cam, and a thermo-quad intake from a late 70's [78 maybe ?] newyorker, plus using a big block 727 case during the rebuild, and using the output shaft and tail shaft of the small block 727. this allowed the original transmission mount and drive shaft to the transfer case to be re-used, and just needing 1/2 x 3" wide plates as spacers with properly drilled holes to bolt to the modified 318 block mounts and the factory frame mounts.
just changing the engine went from about 8mpg to 14 plus a couple of points.
i chalk that up to the 440 not working as hard to pull the weight of the truck compared to the 318.
unfortunately, i had to sell that truck to use the money for the paperwork needed when i bought my stupid house.
on a bright note, however, the family that bought my truck still own it today. it has been used as a pulling truck, a mud truck, and a street truck in the time they have owned it, and it still retains the engine and transmission i installed so long ago, as well as the "basher bumper" and bed i built.