I am going to have to guess the problem here is with using the factory installed cross-bars. Like others have said, make sure they are as far forward as possible. I have the factory installed side railings with a thule system but have been able to use both the Thule Echelon and Yakima forklift...
Just read through the post on this thread and I am quite surprised as to what you guys are posting. Before I disclose my MPG's I will say that my past two cars have been a modified mustang putting 400hp/430torque on the wheels and a Cx-7 Turbo (Yeah clear sign of getting old, I just keep hacking...