Detroit’s automakers, on track for their best sales year since 2006, may want to brace themselves for rockier times ahead. Over the past year, GM and Ford share prices have lagged the overall market, in spite of moves by those two companies to give more cash back to shareholders. Optimists include Kurt McNeil, head of GM’s U.S. sales operations, who said Friday that the industry is on track to have its best sales year since 2006.