I jut bought a 2007 STS with a V6 engine and the luxury performance package and I only get 23 MPG on the highway yet my previous car, a 2005 CTS with the same engine and final drive got 27 MPG on the highway, yet the 2007 is only 300 pounds heavier. Can anyone tell me what kind of mileage they get or give me some kind of insight on why I'm getting worse mileage