Why dirty cars cost more at the Pebble Beach auctions

The most-sought after accessory to any vehicle for sale in this year's Pebble Beach car frenzy isn't mechanical or even man-made. It's dirt — namely a natural layer gathered over a minimum of a decade as a vehicle moldered away as some forgotten family heirloom. In the strange world of rare vehicles, even neglect becomes a valuable commodity.

The cult of the "barn find" has grown to such a state that it's no longer sufficient to just say a vehicle emerged from an old shed. Washing the dirt off a stored car takes away a piece of evidence that's increasingly valuable to collectors; without that dirt, it's easier to raise doubts about what kind of repairs or botched restorations might have been attempted in the past.

At the RM Auctions on Friday, a 1965 Shelby 289 Cobra that had sat since 1987 was offered just as the seller had acquired it from the previous owner's estate, with dirty tires, trunk and engine compartment. It sold for $720,000 Friday night.