Poor Mileage

I have a 2000 Alero, 3.4L with 165,000 miles. I am not sure if it is from the number of miles, the remixed winter gas, or some combination of problems, but I am no longer getting the 30 mpg that I used to get. I have recently been getting a consistent 25 mpg. The plugs and wires were changed at 100,000. The wires were junk and replaced again with 8mm wires at 130,000. She runs well and is pretty smooth at idle. I have had a few problems over the past 2 years that I eventually root caused and fixed thanks to AF (problems with excessive oil from a K & N filter on the MAF sensor and a bad air pump). Still, the poor mileage persists. Now I just got a EVAP small system leak code, but it doesn't sound like that should be the problem. I have also tried Sea Foam in the tank. My next thought would be new plugs and if that doesn't work, a professional fuel injector cleaning. Does anyone have any other ideas of what I might try?

Could be your O2 sensors, your still getting better mileage then most 3.4 owners.

What type of plus are you using now ?

On the Evap make sure your gascap is tight, which you probly knew. Also the hoses under the rear by the fuel tank and where a evap purge valve is located. The hoses sometimes become loose and leak , tighten what seems loose.

I don't remember the brand of plugs, probably AC, but they are the same style platinum plugs as OEM. Thanks for the tips on the EVAP. Need to educate myself a little more on that system. Not getting any codes on the O2 sensor. Could they still be defective?