I read an aticle in the past weeks Houston Chronicle auto section about the 500. It is inbetween the size of the Taurus and the Crown Vic on the outside, nearly Crown Vic size on the inside, and a trunk the same size as the Town Car with a weight of about 5500 pounds. This article listed the 3.0L Duratec as the only engine for the car. That didn't seem right to offer a car of this weight with the 3.0 as the only engine, offering the 3.5 as an option makes more sense, but then again, it is Ford.
