You should inspect your car's TPS, MAP and Oxygen sensor (Bank 1, Sensor 1). They should be in brilliant shape for the car to give good mileage. Also, a dirty or clogged air intake filter will destroy your car's mileage. A worn out transmission causing sluggishness can hurt your car's mileage. Bad or improper sized tyres can affect mileage. Wrong spark plugs can affect mileage. Leaking fuel injectors can hurt the mileage. Even bad ECM ground connections can hurt your car's mileage by adding electrical noise to signal outputs which causes ECM to read things wrong which eventually can hurt mileage. You see, there are several factors. Address all these one by one and i'm sure you'll be able to figure out what's wrong in your case.