My father actually gets fuel mileage in the upper teens with his half ton (I've never really measured it precisely), and my half ton averaged about 18MPG when he was driving it (I did measure that one).
This one ton truck sits up higher than the half tons (more drag), has deep gears (runs 2,900RPM @ 65MPH), and @ 4,500lbs it's pretty much 1,000lbs pounds heavier than a half ton. I'm sure I can/will get a little bit better mileage out of it just like it is if I try (less fast driving/traffic/WOT), and I'm expecting that adding an overdrive at some point will make a very noticeable difference under most driving conditions; even with the little 250, I could pull another gear a lot of the time when I'm currently stuck in 1:1 with the engine wound up.
I do have a '68 Impala for an aerodynamic/lightweight "economy car" (
) to go with the one ton, though; expecting/hoping to get over 20MPG from that one.
(I was going to use a half ton suburban or a truck with a camper top, but I figured the actual car would probably be better suited as a car...
)