O.K....Apparently not too many GXP owners care about mileage (or, perhaps there are just not many members of this forum).
How about water infiltration in the roof? After the last rain I thought that I heard what sounded like water sloshing in the roof cavity in front of the sun roof. After making a sharp turn or two the sound went away or the water must have drained away. Should this concern me?
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2006 Grand Prix GXP, Black Cherry