A horse! No gas to buy (after the apocalypse, who's going to be "selling" it), plenty of forage for fuel, and all you need for "spare" parts is another horse of the opposite sex.
And instead of a nitro injector, just flashing a bottle of Elmers glue is sure to exrtract that extra burst of speed.

FQ13 who still says that if you can't pedal it, saddle it or row it, its not a post apocalyse vehicle