Wasn't that the Exxon marketing pitch way back to show how great their gas was? 1980s or 90s???

Anyway, for quite a few years I have been brand agnostic when it comes to gas for my vehicles. I have kept track of my MPGs and I never really saw a difference in MPG I couldn't attribute to some effect (long distance driving, cold weather, or some such). That is until this past month. I ended up buying gas at a regional convenince mart / gas location that I hadn't gone by in years (I just happened to be in the area). The gas cost about 8% less than other stations in the area, so I quickly filled up. After that fill up I noticed the fuel guage going down quicker than usual. I was concerned I had a problem with the car, but then I remembered I had bought gas at an unusual location.

After the next fill up at a brand name location, I checked the MPG from the prior fillup, and saw the MPGs had dropped 10%. After using up this next full tank, the MPGs went back up to where they had been previously. And have held steady since.

I guess this shows gas is not a universal commodity, there is some difference to the brands, and getting cheaper gas may not be the deal you think it is.