which fuel?????

[WyrreJ]

Isn't regular cheaper not only because it's less octane but because it tends to be the bottom of the barrel or mixes of different grades?

I got bad news for ya. Midgrade is just a mix of the low octane and high octane fuel.

Almost all gas stations have just two tanks in the ground, low and high octane. They mix them at the pump to get the in-betweens. In many cases, you can actually mix it yourself (pump 2/3rd low octane and 1/3rd high octane or whatever ratio it takes) and it is cheaper than letting the station mix it.

However, you are not alone in your misconception, the gas companies actively seek to mislead people into thinking just that, to trick you into giving them more money.

[Ldub]

A lot of the "mid-grades" around here are 10% ethanol to increase octane.
$.02 from ND.

[VehiGAZ]

The 10% ethanol is there to reduce emissions, not to improve octane. The carcinogen (I think) MTBE was the most popular additive until recently, when it started showing up in drinking water supplies. CT and many othe rnortheast states switched over to ethanol very quickly.