Hey all,
I just purchased my '11 aveo5 Automatic a week ago (used) and have ran a tank of gas through it on the highway. about %95 highway driving (probably more) got me 28 MPG. I was pretty disappointed as I was expecting around 34 or maybe even better since I was drafting behind some trucks some of the way. My previous car which was totaled in an accident was a 07 toyota yaris which I would get over 40MPG on the highway with so I know its not my driving style.

Would this have anything to do with my engine sounding pretty rough when its idling? Also, it hesitates pretty badly when you step on the gas. Its like its starts, then bogs down, then goes...

Theres also a rattling or groaning like sound as you transition from idling to starting off coming from the front.

Would this all be something I could take it to the dealership since its still got the manufacturers warranty on it? What would I need to tell them, would it cost me anything?

Would I improve gas mileage by getting "iridium plugs" and new wires?