DETROIT  While its crosstown rivals stumbled through bankruptcy this summer, the Ford Motor Company pressed its advantage, and delivered surprising news on Monday that its cost-cutting efforts and improving sales helped it earn nearly $1 billion in the third quarter.

But now it faces new challenges in maintaining that lead. Both General Motors and Chrysler, with the stigma of their bankruptcies receding, are moving ahead with their own comeback plans.

Under prodding from a new board, G.M. is aggressively marketing a 60-day money-back guarantee on its vehicles that most likely will result in market share gains to be announced on Tuesday.

And Chrysler and its new partner, Fiat, on Wednesday will disclose an ambitious five-year plan to streamline its product lineup and introduce more fuel-efficient models.