I have a Thule Ascent 1500 and it's a great travelling companion to my X5. We did a 3500+ km round trip from Ontario, Canada to Myrtle Beach/Virgina Beach recently and went through a nasty rainstorm most of the way there and the box held up quite well.

You might want to try placing the cargo box as far back as possible. I find it's the best for reduced wind drag and noise.

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2006 X5 4.4i LeMans Blue