I'd be very skeptical of anyone claiming a truck from the seventies, 3/4 ton or larger, 350 with 15+ mpg. I don't think it can happen.
In my experience, you can expect maybe 10 mpg with this rig. Probably won't make any difference between fast, slow, loaded and empty.
3/4 tons are heavier, have heavier rotating components, usually lower rear end gears, and sit higher, having all the aerodynamic properties of a shoebox.
My old '72 C-10 had a 350 4 bbl, three-speed manual trans and 3.08 rear. It could pull down maybe 17 mpg on the highway but only 10-12 around town. A friend bought a new '73 with a 250 six and manual transmission, and he couldn't quite match my mileage.
But I've NEVER seen a 3/4 ton come anywhere close to that. And the '73 and up trucks, as memory serves, didn't seem to do mileage well, according to the owners that I knew.
Maybe some of the later EFI engines with slicker bodies might be able to do a bit better, but not with carbs.
My 3/4 ton '91 GMC Suburban with TBI 350, overdrive automatic and 3.73 rear gears gets around 12-13 mpg. Another friend has a similar '92 with a 454 and he gets about the same mileage.... he just has a bit more reserve power.
