there is no doubt from my experience and what i have researched that adding a roof basket to your vehicle robs some mpg even when empty. not to do with weight but just streamlining/wind resistance. running cross bars and a fairing on a car, without anything strapped down, will drop the mpg even. looks like most people do use them, but if you seldom do and care about fuel efficiency, those are main culprits for losing a few mpg. side view mirrors contribute a lot to wind resistance too obviously...would be cool if there were some oem mini side views available for those who care about that sort of thing - i wouldnt take it that far personally.