I don't know of any gas station owners that fill up their own underground storage tanks. That job is left to the distributor of the oil refineries. So if it is the case that you're getting a different type of gas when you're paying for Tier 1, then you should place blame on those guys who drive the big oil rigs, because they are the ones that pump it into the ground.

Also keep in mind that the quality of gas (which could affect your MPG) varies depending on the gas station you are at. I. E., Acme gas Vs BP.