I've already got roadbooks that go from Vegas all the way to the boarder, about 800 miles, almost 100% dirt. Too bad they couldn't take that route!
With the commonplace long liaisons in Dakar, they could start in the Coliseum in Los Angeles or even the Luxor parking lot in Vegas and make their way to the Mexican boarder where the Special would start. Maybe throw in a short bit of a prolog in the dirt while passing through Primm!
I wonder how many donuts Robby could do between those concrete lion things in front of the Luxor?
I think there's no road through the South part of Panama into Columbia, so they would have to take a boat to get into South America, or finish at the Panama Canal (easy loading everything on the boat for trip back home).