I am looking to increase the gas milage on my wife's expedition. I found that FIPK thing from K&N that adds 20% MPG and 17HP. Would aftermarket mufflers do the same for MPG & HP ? What else could I do? She drives very smooth and rearly stomps on the gas. I am getting tired of getting 14 MPG in the city. I figured may be I could pull a couple more miles out of each tank. Oh, one more thing, will a chip help with that too?