After Leviathan!Sam and Leviathan!Dean go on a killing spree (while driving a 67 Impala) causing the real Sam and Dean to become America's 2nd most wanted criminals, Frank Devereaux advises the boys to lock up the Impala to avoid detection. At first they drive a Pontiac Acadian, then Dean steals a Dodge Challenger. After Bobby's death, Dean drives a Buick Special Deluxe and then, when he's in 1944, a Plymouth Roadking. After making it back to 2012, Sam and Dean drive a Buick Riviera to Seattle and then a Mercury Cougar to Wichita, Kansas.
The pony car they drive in Kansas is a two door 1973 Mercury Cougar XR-7 hardtop with a semi-fastback and "flying buttress" sail panel. 1973 was the last year that the Cougar was based on the Ford Mustang platform. It has plate numbers: 491-D6N.