Hello and welcome to JustAnswer!
There are many things that adversely affect fuel economy.
An inaccurate upstream oxygen sensor will definitely cause the engine to run richer or leaner than it should if the sensor becomes inaccurate.
An inaccurate mass airflow or MAP sensor will cause the engine to run rich or lean.
Leaking/worn fuel injectors can cause poor fuel mileage. This would usually cause the engine to run rough to some degree.
The ignition system shoudl be looked at also. Worn/aged spark plugs and plug wires can lower fuel mileage. Engine performance issues such as leaking valves or compression issues will also cause poor mileage, but again the engine should run rough to some degree.
Having the proper tires, properly inflated, and making sure the alignment is correct will also help raise fuel mileage. Under inflated tires or the alignment being incorrect will lower mileage.
You'll want to check for fault codes also, diagnosing and repairing any codes that are setting.