General Motors Co.'s "Maximum Bob" has apparently reached his vanishing point . General Motors Vice Chairman Robert A. "Bob" Lutz will retire from the embattled carmaker effective May 1, the executive confirmed yesterday (Wednesday). Lutz had been serving as a senior adviser to Edward E. "Ed" Whitacre Jr. , GM's chairman and chief executive officer. The move comes just one day after GM announced another shake-up in the North American unit that's supposed to be heading the company's overhaul.