[trooper Tdiesel]

in short....
fords are the most common to be converted to 4wd
GM is 2nd
dodge is very seldom swapped over.



biggest size van with lowest cost to drive with a OE gas engine and auto trans?
"its the most aro" thus an advantage in that respect.
the 90s/2000s dodge 1 ton 15 passenger with OHV 5.2L 318-V8
they can pull 20+ mpg on the open road, oddly good for there size.

a ford van with the 4.2L V-6 would be close or better mpg but no where as big as far as cargo space as there mostly in 1/2 tons the 3/4 and 1 tons got the V-8s and V-10.
unless you convert a newer gas van to manual trans. that's about as good as it will get for a gas engine.


after that its diesels,

first is this OE type swaped to a manual it would be mid 20s
there was this the 6.2L and later the 6.5L with the van model having a very rare 6.5L N\A set up in some cases.
Compare Old and New MPG Estimates

i know ford diesel vans are out there also, but being 1 tons there not listed on the mpg site...

after that there's not very many OE diesels.
putting a 80s ford 6.9L or 7.3L IDI N\A with manual trans in to a 90s dodge van.

or a cummens 4BT or isuzu 4DB1 or 4DB2 all are 3.9L, around 750lbs 100/130hp, with max rpms of about 2,800
with a manual trans that could push the number up in to the mid 20s or just in to the high 20s when empty and driven slow.
a direct drive trans with a final gear in the 2.73/3.55 range would help up the mpg. over drive trannys wast more then a direct drive trans does when in high gear. for as long as you have a final gear that makes up for not having the O\D gear.