There's no guarantee that a Garmin (or any other add-on nav device) will give you the absolute best routing for a given trip or situation. For trips of significant distance, I always do some reasonability checks on the suggested route (even when using Google Maps).

Load the Nav DVD, leave it in there and try it out.

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2008 X5 4.8i, Jet Black, Tobacco Nevada leather & just about everything else (no vent. seats, no 3rd row)