Your welcome, but back to OP's question I don't have a answer for you but I'd quit running premium.

 

And how long has this drop in mileage been going on? You mights want to look into faulty plugs or cop.

 

Check tires pressure? My truck gets around 11mpg city, 2001 supercrew 5.4l 4x4.

 

mine gets about the same 11.7mpg 2003 supercrew 5.4L FX4

 

A thought to consider,
I've found my mpg decreased slightly when using 10% ethanol gas. There's only a few places left locally that still carry ethanol free gas and I fill up there. My truck seems to like it better as well. Although the ethanol mixture doesn't hurt my truck I prefer to get my money's worth....

I would start with emissions and work your way back to fuel delivery.

It could be a number of things... Driving habits, vehicle modifications ie: lift and tires, terrain, amt of cargo etc etc. Has the service engine light come on / gone away?

Bad EGR valve?
Bad O2 sensors / dirty air filter?
Gas leak around injectors due to bad o rings?
Prolonged oil change and oil viscosity?
Bad fuel pressure regulator?
Damaged/old/corroded spark plugs?

 

My mileage has been not so great the last couple weeks as well. I live south of the bay area. I think it has to do with the winter gas change over. I run safeway gas most of the time and 89 octane. If you have the cash, I would recommend a fun toy for your truck, the scangauge E or scangauge 2. I have the E one and with the differing displays, I can see how moving my foot a little lighter on the peddle can make some improvement.