I'm very puzzled.

My mileage has dropped 2-3 MPG even though my driving habits haven't changed.

It was time for plugs and wires, so I went with Beldens and NGKs. I checked engine vacuum, normal, and did a compression test, all cylinders right around 155. Oxygen sensor about 10K ago (Delco) which resulted in no change in mileage.

Any ideas, guys, besides a midnight stakeout of the vehicle with my 12-gauge?

Do your fuel blends get changed around this time of year like they do in my area?

Changes in the fuel blend can seriously effect MPG.

Take for example a trip I took to NY a year ago. Way there..mileage was great. Way back...mileage sucked and I drove slower and better..mileage wise.

NY Gas is known to contain mileage and power robbing ingredients .lol

California has oxygenated gas (10% alcohol) year-round, but the car has always gotten 20 MPG with little difference between highway and city driving since it was new.

Guess that makes sense, though, I'll check it out.

Air filter, tire pressure, lack of wax can be factors.

Lack of wax? :?:

I can't wait to hear this one!

Less resistance don't ya guys realize...

That'* slowing you down ever so little.

Right. Which explains why military aircraft are commonly painted non-gloss, and why nobody that runs at the salt flats waxes for speed?

Ok, we had our fun. Back on topic. :lol;

Its probably like Boosty said the gas formula changes I think Sept 1 to the winter brew.
Have you run fuel injector cleaner lately?

wow that would make since! My plugs and wires were totaly bad so I thought that was my MPG problem. I changed them and it went from 20 up to 22 mpg but that is 3 mpg lower then I got all summer. and yes I just started noticing the problem with the colder weather in september and the damp dew in the morning is how I found out my plugs wires were bad.
wInter blend gas has MPG robbing chemicals in it?