* Engine size: C10s came with a wide variety of engine options, from inline-sixes to big-block V8s. A six-cylinder will get significantly better gas mileage than a 454 cubic inch V8.
* Transmission: Automatic transmissions generally consume more fuel than manuals.
* Vehicle condition: A well-maintained engine and properly tuned carburetor will yield better fuel economy than a neglected one.
* Driving habits: Aggressive driving significantly reduces gas mileage.
* Payload and towing: Hauling heavy loads will dramatically decrease fuel economy.
* Tire condition and inflation: Under-inflated tires increase rolling resistance and reduce mileage.
While it's impossible to give a precise average, you could generally expect anywhere from 8-15 mpg depending on the factors above. A smaller six-cylinder engine, well-maintained, and driven conservatively might approach the higher end of that range. A larger V8, poorly maintained, and used for towing or hauling, could easily fall to the lower end or even lower.
