Went from about 300 miles per tank to getting 150 after changing sensor.

Sensor is correct part #
I noticed an issue before- now gone- where after my car was warm, RPM's would creep high- in the the 1000 to 1200, sometimes 1500 range when I would shift the auto tranny from D to N, for instance.
Now that's gone and the RPM's seem settled at about 800 while in gear and stay that way when shifting around from P to N to D etc...
Exaust seems to smell awful now. Before new O2, seemed better. No clouds of smoke or anything.
AMM has been working fine.
Plugs have maybe 8-9 k on them now
Cleaned IAC recently.
Cleaned TB w/new gasket recently.
Gas gage seems to work fine. (Temp gauge does stick occaisionally, requiring a thump of the dash board)
Tank does not seem to be leaking anywhere.
Both fuel pumps seem to work.

Other than someone siphoning gas from my vehicle (possible), I don't know why I would see this major jump. And at $4/gal it's pretty evident this can not continue.

Where to start? Thanks guys...
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89 245 'loaded' with a Great Pyrenees