The octane rating has nothing to do with quality of gas, that's more of a brand specific thing.All the octane rating is for is to show how easily the gas combusts. The higher the rating the harder it is to burn the gas, so it only ignites when the spark is fired, instead of pre-igniting, which is bad.
High performance engines, se-r, have higher compression which causes pre-ignition easier on the lower octane fuels.
Mileage "gains" you see with different fuels are almost impossible to substantiate as conditions are never identical. I get from 28 (all city) to 41 (hwy) and no two tanks even on similar runs are ever near identical.
One thing you may notice, is running 85 octane, like I do here in Wyoming. If you fill up with 87, the engine idles smoother as it doesn't need to retard or advance timing as much to compensate for the lower octane.
If you fill up with premium or even mid grade for most of you guys, you are simply throwing away money.
Now my Hemi Dodge, that needs 89 or it runs like s**t! But again, no improvement over 91.
