Cadillac Repair: 1991 caddi deville, poor gas mileage, fuel mileage

Question
poor gas mileage.  I changed spark plugs, disturber and new coil and filter.  I'm
only getting between 12-13.9 mpg.  What I'm I missing?

Answer
IS or HAS the CHECK ENGINE light ever illuminated on the dash while driving the vehicle.

Was any type of work of any kind performed on the vehicle prior to this low fuel mileage ?.

Does the vehicle idle OK ?.

Does it accelerate as it should ?.

When was the last time you checked your tire pressures ?.