Okay ... as an example, here's what happens in the DC area:
http://www.washingtonpost.com/local...-investigation/2011/05/13/AFUtrx5G_story.html
Basically one distributor for the entire area. Anti-trust and price fixing issues aside, pretty much all the gas stations around here get their gas from the same pipeline and distributor. This arrangement is the case more often than not in many parts of the states ... sometimes a couple of distributors will compete but there are almost never, as I understand it, 1 distributor for every 1 fuel brand name.
Now, you wanna tell me that you're seeing
significant mpg variance between different fuel brands simply because they have a slightly different mix of heavily regulated additives, so be it ...
The only way to really know is by filling up, putting the truck on rollers, set the cruse at a constant speed, and see how much fuel of each brand was used over a constant time frame.
Seems like a lot of effort though.