* Driving conditions: City driving will yield significantly lower MPG than highway driving. Stop-and-go traffic drastically impacts fuel economy.
* Vehicle condition: Properly maintained vehicles (tuned engine, properly inflated tires, etc.) will get better gas mileage.
* Driver habits: Aggressive driving (rapid acceleration, hard braking) reduces fuel efficiency.
* Payload: A heavier load will decrease fuel economy.
* Tire size and condition: Larger tires or tires with low pressure will negatively affect MPG.
That said, you can expect somewhere in the range of 14-18 MPG combined (city and highway) for a 1991 Chevy 1500 4.3L V6. Highway driving might push it closer to the higher end of that range, while city driving will likely be closer to the lower end or even lower.
It's difficult to give a more precise number without knowing the specifics of the vehicle's condition and typical usage. The best way to determine the *actual* gas mileage of your specific truck is to track it yourself by filling the tank completely, driving for a while, then refilling the tank completely and dividing the miles driven by the gallons used.
