Not just the car, though, but because it's such a large country it's got the never-ending highways, and the sheer variety of immigrant populations makes it easier to legitimate non-American myth and lore. I've always been charmed by their using smaller cities and towns as locations rather than the usual NYC or LA. A lot of the classic rock, too (although Zeppelin came from London), the Southern/Western-style dives, the pop culture references...I dunno. kroki_refur did an amazing crack British!AU called Winchester, Like the Bishopric (http://kroki-refur.livejournal.com/274186.html) about this, which is honestly what got me thinking about it:

"Yeah, so, I wrote this after one of fandom's many small kerfuffles about Britishisms in fic and how the American-ness of the show is so very central to the whole thing. I'd originally planned to write an actual ghost hunt -- because it's not like we're short of the wee buggers in the UK -- but the story took on a life of its own and decided it was going to be about something entirely different. And then Sam and Dean ended up going to America, so maybe what I learned is that the American-ness of the show really is central to the whole thing. Then again, maybe I just learned that we may be divided by a common language, but the language of pie is universal."

So, yeah. Anyway.

YOU HAVE AC:C AND HAVEN'T WATCHED IT. THIS IS MY 'JUDGING YOU' FACE.