Interestingly there was at least 2 cars used as Biffs.
There are continuity errors in the film with the two small rectangular fog lights on the bumper, the super delux badges on the bonnet and front wing.
http://www.jackincongruente.com/wp-cont ... e-Biff.gifOne car is obviously a super de-luxe (with all the extra badges) the other is a standard drop head coupe.
There may have been more than 2 cars (particularly with the tunnel wall scraping and the side swipe manure scences) who knows for sure?