Martin Winterkorn’s days as head of Volkswagen AG appeared numbered on Tuesday after the German carmaker said a scandal over falsified vehicle emission tests in the United States could affect 11 million of its cars worldwide. The Tagesspiegel newspaper, citing unidentified sources on Volkswagen’s supervisory board, said the board would decide on Friday to replace the 68-year-old with Matthias Mueller, the head of the automaker’s Porsche sports car business. A Volkswagen spokesman denied the report.