I have a 2001 f150 4x4 with the 5.4. Just turned 141xxx and the last couple weeks my gas mileage has been getting worse and worse I have less and less power and I don't get any power until I'm over 3500 rpm and even then there isn't much. Oil changes regularly every 3000-5000 miles. Around 120xxx it got a new intake, new coil packs, new plugs. Up until about a month ago I was getting 14-15 mpg in town now I don't get that on the highway. I have no check engine light, and it doesn't miss fire. Runs great. Just doesn't have any power

 

I would scan for codes anyway...sometimes the CEL light won't laminate with a code stored...its suppose to...maybe your CEL bulb is burnt out?

Sounds more like you got a vacuum leak some where? WAG

Are you sure you don't have a brake hanging up or E brake pads lost their moorings...maybe a bad wheel bearing/s...point is many things can account for poor MPGs .

Are you low on tranny fluid?