based on the 1st and 2nd gen 12v's that I've owned and a few I've maintained, no, that's hear-say. even a manual trans with a gear vendors would be nearly impractical to drive with higher than 3.55 gears. I'd believe a consistent 25mpg with a max effort 2wd mileage truck, it'd be difficult and take some seriously attentive driving technique but I don't think is impossible, especially with a rotary pump.
funny thing is my dad's '91 cab/chassis with 10k lbs of utility bed gets 17mpg while my flatbed '93 gets 17-20
slightly different throttle application...
it would be interesting to me to see a max effort 2wd, though. Lowered, small tires, minor engine mods, light weight synthetic gear oil...but at that point, I think I'd rather have a C10 or early 70's f100 with a 4bt and get 35mpg in a smooth riding bagged 2wd truck!