I got a 2007 F150 with the 5.4L. I had true dual straights put on last year and had just changed my plugs about a month ago. I used to average around 17 mpg in the first 2 years of owning up until the last 5-6 months i dropped down to around 7-9mpg. Other than the exhaust the only things I have changed is i threw in a k&n filter and went up to 265/70s from 255/70s. my bank 1 cat code just came on the other day but I can't seem to find an explanation for the ****ty mileage before that.

 

Sounds like pluged CAT(s). My driver side one got worse and worse the longer I drove it. Fuel economy droped to around 10mph for me. You should have cut them off before you put on your true duals.

 

Umm, no. Technically illegal and you won't pass an emissions inspection. Just those mods aren't going to cause them to plug up, something else is going on here that's causing it to run too rich.

 

I also changed the fuel filter around the time my plugs were changed. I thought it could be something with my o2 sensors but they are running within normal range. I also cleaned my MAF with a can of CRC and cleaned my filter a few times.

 

If you do discover it was your cats I highly recommend the Magnaflow 94106 universal cats. Legal FTW