i agree with Zblackbeast i would check your crankcase ventilation and the pcv valve, escapes are also equipped with a DPFE (Delta Pressure Feeback Sensor) which can get clogged up and cause poor gas mileage and if it does fail, it will throw a soft code which will not cause for a check engine light to come on, but if you use a diagnostic computer it will show a EGR Flow insufficent and or overflow. Also i recommend a bottle of AMSOIL P.I. Performance Improver you mix it with your gas and its good for 4,000miles and i went from 18mpg to 24mpg with a bottle of that stuff and havent put another bottle in and im still seeing good results. | 
