Another place for a possible EVAP leak is at the fuel pump module O ring. Did you lube it before installing the module? Did it seat completely without getting distorted when tightening the retaining ring?

Very few EVAP codes will cause any noticeable difference in performance or fuel mileage. Especially not the P0455, unless fuel is leaking out

If the distributor has been out, and camshaft retard is not within specs, that could cause a slight drop. With colder weather approaching, it's possible the drop is normal as the PCM keeps the fuel mixture rich longer during warm up... colder ambient temp equals longer warm up time. Might want to jack up all 4 wheels and make sure that no brakes are dragging.

EDIT: make sure the fuel fill cap gasket is good, no cracks etc.

 

Thanks for the helpful tips Captain. The cold weather may be the cause, since I do all my commuting at night. I haven't messed with the distributor since I've owned the truck, but even so I need to adjust camshaft retard, last time I checked it was off a couple of degrees. But the MPG drop was pretty sudden, so I don't think that's the cause.

I didn't know about lubing the seal. If the filler neck isn't the culprit, I'll check it when I drop the tank. One thing that I thought of, I bought a new retaining ring from Advance this time around since the original was pretty crusty. Hopefully it's sealing properly.

 

On a 2 door, you don't have to drop the tank to replace the filler neck. Remove the wheel and plastic splash shield and you can get right in there.

 

Oh yeah I know, I just meant if I replace the filler neck and the code is still there, I'll be dropping the tank to check all the other possibilities.

 

Got around to checking things out to fix this P0455 code. I'm an idiot and just didn't seat one of the vent house connectors firmly enough when replacing the fuel pump. Had to partially drop the tank to get my hand in there. After driving around for almost a week waiting for the EVAP monitor to complete, tonight it did and still no SES light. Success. Thanks for the help.

 

Stuff happens Glad you got her fixed, and thanks for posting back.