GM-GMC: Increased gas consumption, gasoline engine, gas mileage

Question
I don't know if you can help but my dad wanted me to look on the web and find out why all of a sudden his gas mileage is decreasing.  He has a 1995 Chevy Cheyenne 1500 truck with a 5.0 gasoline engine.  He has replaced the gas filter and the air filter even though the air filter wasn't that dirty. He was getting 21 mpg then it dropped to about 17 and now it is down to 14.  Tires are inflated properly and he has checked out everything that he knows could cause a decrease in gas mileage such as the items mentioned above. Thanks, Brenda

Answer
How long has it been since the vehicle had a tune-up?

Spark plugs wear, spark plug wires go bad, and various settings can stray out of tolerance.

Is the vehicle being driven differently?  If it was driven mostly on the highway and is now used for city driving, the fuel economy will be lower.

Winter?  Cold weather causes vehicles to use more fuel.