I only have the roof rack (ATOC Tandem Topper) because I need to transport the tandem, but I also use it for the fixie when it's got fenders on. Otherwise, the fixie goes in the trunk of the Jetta TDI with both wheels off. Much better fuel mileage.

When I drove to Davis, CA, we put my friend's bike on the roof rack and I was getting 36 mpg (diesel) along I-5. After she flew back home, I drove to Borrego Springs (US 395) and then back home on I-5, with nothing on the roof rack, and I was getting up to 45 mpg. So all that drag on top does make a difference. Had I taken off the roof rack, I likely would have got close to 50 mpg. (Yes, there is a deflector on the front of the rack. It is REALLY noisy without it, especially with the sunroof open!)

Luis