I don't know about the battery, but from experience I do know that when I crank up the volume on some tunes that I refer to as "drive music", I have a tendancy to get a lead foot in a big way and also temporarily forget about maximizing mpg, so I guess you could say that a nice stereo system could affect your mpg. Someone should do a study on that...measure mpg for a 45 minute drive with total silence versus the same drive with a U2 or Stones CD at volume level 8. Betcha there would be more mpg during the silent drive...Hey, I just thought of something...I wonder if I can increase my mpg by listening to Mozart during my commute??