Do you have the LT2 (that's what your profile says)? The reason I'm asking is the LT2 only comes with the 6 speed transmission. You mentioned you have the 4 speed. The 6 speed does better on the highway than the 4 speed. The 6 speed is rated 3 MPG better than the 4 speed on the highway. The 4 speed is rated at 30 MPG and the 6 speed is rated at 33 MPG. The city MPG is identical for both transmissions - 22 MPG.I just got home from driving from Lafayette, LA to Hoover(Birmingham), AL. My 2008 Malibu 4-cylinder/4-spd auto got 32.6 MPG and that was with the cruise control set at 76MPH almost the whole way, it was a 440 mile trip. The car had me my wife, our 2 dogs and trunk full of stuff too. I ran 93 octane in it the whole trip and I have put a K&N airfilter in it, not sure it that helped any. Just wanted to share, thought that was awesome.
But getting 32.6 MPG is very good - especially for a pretty heavy car. I've gotten over 33 MPG on the highway.
I noticed something interesting that I was unaware of. I found the Chevy Malibu Website for 2009 and it says that the LT1 had the 4 speed as standard equipment and the 6 speed was optional. I didn't realize that. I thought the 6 speed was always standard on the LT1.