Some
Pyramid issues may be worth checking out. First there's #3/03, Venturing Into The Badlands, which is of course dedicated to post-apocalyptic gaming. There may also be helpful stuff in the Tech and Toys releases (#3/12, #3/37, #3/51) and Vehicles (#3/40). GURPS 4e is sadly a little sparse on
detailed vehicle design rules, but that and the forums should cover your needs if you plan on going Mad Max with the game.