That depends... 2.2L or 2.4L? Many of us get better mileage with premium in the 2.4L, and it is recommended. Not necessary in the 2.2L as it can't use the higher octane.

But! It could be a bad tank of gas

 

: are you telling me i wasted money on a whole tank of premium?! i have the 2.2

 

Not so much wasted, you just gave the engine a treat. The 2.2 can take a slight advantage of Premium fuel as the knock sensor will allow the engine to get a bit more aggressive with its timing. I give my 2.2's a treat about once a month, to get a different additive package through the injectors, its always good to change flavors every once and a while. Running Premium won't hurt anything, you just won't see a big performance gain because of the state of tune on the 2.2 engines.

 

I figured as much since you posted this in the 2.2 section...... The new station you went to could have bad tanks and that would give you bad gas......

 

Perhaps the gas you usually fill with is straight gasoline and the station you went to for the last fill up has gas that is 10% ethanol..... will hurt your mileage! Here in western NY that crap is mostly all we get, though I have heard from regional customers I talk with that their area doesn't have it, or they can travel across a state line to get real gas and do get much better mileage.
This still won't account for that big of a difference, but could account for some of it.