How are you measuring your gas mileage? Old fashion miles divided by gallons or are you using the dash readout? Do you always fill on the side stand or center stand, or do you mix it up? Do you always fill the tank up to some consistent visual reference point like the bottom of the filler strainer?
Any shop that does emissions testing can give you a reading for short bucks. A probe inserted at the end of the exhaust, with a fully warmed up engine should show 1.5% CO or less. While this isn't a root cause diagnosis, it is a way to confirm that there actually is a problem.
I really doubt that the O2 or air flow sensor are responsible, if they were that far out of range you should have a diAG code set and possibly a check engine light. Codes set?
Assure us that you haven't added something like a Power Commander or adjusted the CO levels...
High on the list of suspects would be one or more fuel injectors not fully closing. A few tanks with Sea Foam or Techron may clean the injectors. The injectors can be pulled out and flow tested, but before doing any of that pull the plugs and see how they look. Soft fluffy black deposits are a sure clue to rich operation. Since you just changed your plugs they should be pretty clean. Do you have the old plugs to look at?
Possibly the ECU thinks that the engine is cold and is staying in the cold enrichment mode. Again, check the diAG readouts for temperature readouts. A quick, but not conclusive test -- when the engine is fully up to operating temperature, in first gear accelerate briskly then chop the throttle off. If you hear pop, pop, pop in the exhaust the ECU thinks the engine is cold and has the Air Injection System active. The AIS isn't the cause of the mileage problem, that fact that the ECU has the AIS active may imply that the ECU is running the cold enrichment FI map. This would also cause the perception that the engine is running quite smoothly and may have slight bogging on acceleration. In fact, any excessive richness, regardless of root cause will cause these two symptoms.
One other silly thing you can check is to see if you have a fuel leak/drip. The root cause of this could be an excessively hot gas tank or a tank venting problem causing dribbles at the overflow tube. Do you smell gas when parked in an enclosed space like a garage?
