I travel back and forth between Pensacola and Atlanta a lot and recently had a chance to compare the gas mileage on a new car with two bikes on a rack behind Vs no bikes. The car is a Nissan Altima, one of the seemingly interchangeable lozenge shaped cars so common these days. The reason recent design cars are all so seemingly similar in exterior design is because fuel consumption is now a VERY important element to the popularity and sales of a vehicle and the "lozenge" shape works.
Last week I had occasion to drive to Atlanta with a "slick" car and come back with two bikes on a Saris rack. With cruse control set right at 77 mph both ways and the trips timed to keep me as clear of the Atlanta traffic as possible, I made 34 mpg slick and 29 mpg with the bikes hanging in the slipstream. I knew there would be a difference but was a little surprised that it was that much. The car that I moved out of (Infiniti I35) got pretty close to 25 mpg, bikes or not.