you can have the rear o2 sensor dropped in the pcm program. You should not lose MPG's with the FWI.......has your mpg's dropped, and if so, how much.......your dp have a hi flow cat or no cat? this code may be affecting your mpg's.......how new are your plugs? they should be copper, not platinum, and they should be replaced every 10,000 miles at least.........we need more info.......