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, a Mercury Cougar, and then a Pontiac Firebird.
The Pontiac Firebird Trans Am is a 1977 or 1978 model with Idaho plates: SD 4420E. Dean cares about its condition, expressing concerned that Jeffrey's dog will pee in the car when they have to leave him alone in the backseat.