2000 magnum

I don't think it's possible to get decent mileage from a 5.9 . I have one in my Dakota , gets 13 mpg on a good going down hill with a tail wind . I put in a mileage program and that was a joke . You realize that as the engine wears the mileage goes down . the 5.9 was never designed as a mileage motor but as a sorta performance piece used for pulling a load . A 5.2 is more suited to the mileage issue . I have gotten 20+ with my 5.2 on flat highway with cruise control set at 55mph . When the 5.7Hemi came out it was a game changer as far as performance and mileage is concerned . It's just the design , good luck