For some reason people have the expectation that a C gets better mileage than an A. Often that is not the case, they are around the same numbers. The C has poorer aerodynamics. The A has better aerodynamics, but much greater weight.
You might do a bit better with a rig without the overhead cab bed. They're a bit more steam-lined and might get you 1-2 mpg better, depending on how big you go.
How fast you drive makes a HUGE difference depending on your rig. Slowing from 70 to 65 or 65 to 60 can make a 20% difference.