* Engine size: While it was a 6-cylinder, the exact displacement (e.g., 3.0L, 4.9L) significantly impacted fuel economy.
* Transmission: Automatic or manual transmissions affect gas mileage.
* Vehicle condition: Proper maintenance (tune-ups, tire pressure) dramatically impacts fuel economy.
* Driving habits: Aggressive driving significantly reduces MPG.
* Payload and towing: Hauling heavy loads drastically lowers fuel economy.
* Terrain: Hilly or mountainous terrain reduces MPG compared to flat areas.
Therefore, there's no single definitive answer. However, you could expect EPA estimates to be in the range of 14-18 mpg combined (city and highway) for a 1989 F-150 with a 6-cylinder engine. Real-world mileage could be lower, especially if the truck is not well-maintained or is used for heavy-duty work. To find a more precise estimate, you'd need to know the specific engine size and transmission type and consult a contemporary owner's manual or a fuel economy resource like the EPA's fueleconomy.gov website (though it may not have data that far back in detail).
