I have not yet tried our Thule Atlantis 1800 on my E53, though I have OEM bars for it, we have used it on our Audi C6 A6 Avant. We did not notice too much difference; maybe 1 or 2 MPG decrease. They are quite aerodynamic. I believe the thing to consider is how much overhang you have over the front windscreen. The more that hangs over the front, the more noise and the great negative effect on mileage. Of course, you have to make sure that the rear doesn't impede the upper hatch from opening so you want a box that fits well between that space.

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