Quick comparison to add to the list:
My truck: '01 Ram, reg cab, short box, 5.9L, 4WD, 33 inch tires, off road package (4.10 gears) and it gets on average 14-16 mpg.
My friend's truck: '06 Ram, reg cab, short box, 3.7L 2wd, tiny tires, and he gets the same gas mileage as I do.
You would think mine would be getting much worse, but that 3.7L has so much of a load on it to keep that truck moving it just sucks gas.
A smaller motor doesn't always mean better gas mileage, alot of it has to do with the vehicle itself.
350 in past 'Vettes got good gas mileage, the same motor just not as powerful got worse gas mileage in trucks. Same idea.