i read a few years ago that (most? all?) gas stations switch to a winter blend which loses fuel economy. same with ethenol blends. there are also fuel stations that would throw a code in my avenger and my buddies sebring. after a tank of different fuel, the light would go off. i would also notice the smell from the exhaust was different... worse. never checked fuel economy though. the bad one was co-op. fill up there and light would be on within 20km. never used safeway gas.
X2
I cant say about the co-op gas, but the ethanol blend absolutely hurts economy, and "winter fuel" seems to burn more fuel. Best fuel I have had luck with this winter is Shell for mileage.
I read somewhere awhile back that all the oil companies that sell fuel in Canada must have an average of 10% ethanol blend. Further reading indicated that not all gas stations used E-10, nor do many indicate that they have it when you stop to fill up. Also, it appears that a general rule is premium fuel has 0% Ethanol.
The ethanol blends have "less energy" per litre, so they burn more fuel, and that is why vehicles with the capability to run E-85 burn a significantly more fuel than when they burn E-10 (or regular fuel).