Steve,
The '71 9500 that I drove was a 20 Ft Flatbed Dump Tandem. We had our own gas pumps at the yard and I didn't pay a whole lot of attention to mileage, but I remember the mechanic telling me that it got about 3 to 5. Refilling the truck was also my job and I don't remember the truck using an unreasonable amount. There again, the truck was fairly new. I would definitly rebuild the carb to expert precsion and super tune the engine, if you have mileage problems.
Bill