IMO the 4.6 has passed the initial criteria of still looking good. As far as X5s go the 4.6 in some combinations of interior and exterior colors and fully loaded are quite rare. The excellent ones will at least maintain value. That should drag up value of the other 4.6s that are in excellent shape regardless of combinations. However, X5s don't have a first car emotional connection, significant demand or racing history. I would never expect any appreciation to be enough to view any X5 as a good investment.

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X5 4.6 2002 Black Sap, Black interior. 2013 X5M Melbourne Red, Bamboo interior
Dallas