What turns Chris from a rubber tramp to a leather tramp?

Leather tramps are vagabonds without a vehicle, who thus have to hitchhike or walk. rubber tramps are vagabonds who have a vehicle. Chris decides to abandon his old Datsun so he can experience his adventure in a more tangible way. He did not want to free his stuck car: he wanted to travel unencumbered by unnecessary possessions.

With the battery dead there was no way to get the Datsun running.... Instead of feeling distraught over this turn of events, moreover, McCandless was exhilarated: He saw the flash flood as an opportunity to shed unnecessary baggage.