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I'm hoping somebody can help me out.  Quick history...

Was getting less than normal MPG, by about 4 MPG
Engine light came on, changed plugs and wires, fixed issue
Still getting crappy MPG
Needed new brakes, changed those
Needed new tires, replaced those
Still getting crappy MPG
Decided to change Fuel Filter...its in tank, so no go on that
Oxygen sensor light came on after fill up of gas
Removed front Oxygen sensor, cleaned it
Light went off after a couple tanks of gas
After a couple tanks of gas, finally got normal MPG
On next fill up, back to same of lousy MPG

Ordering new PCV today.
Going to clean second Oxygen sensor this weekend.

Does anybody have any other ideas as to why I'm not getting the normal 26MPG...I currently get 20-21MPG.

Car runs smooth, no hesitation, stalling, nothing.  

72,000 miles, 2005 Hyundai XG350L