Help! My Fiance is killing my MPGs!

I obsessed over this for a while too. Then realized I was being silly and now I just let my wife drive the way she likes to drive, and sit back and enjoy the moments I am actually a passenger as opposed to a driver. I drive most of the time anyway, so it's always fun to see the mpg creep back up after she's been behind the wheel and I take the driving back over. : )