FRANKLIN, Tenn. — Slumping Nissan Motor Co. is offering voluntary separation packages to many of its U.S. workers as it tries to resize itself to match lower sales. The Japanese automaker wouldn't say how many employees it is targeting to leave. It also wouldn't say how much money it expects to save, or give details of the severance offers. The offers will go to factory and white-collar workers over the age of 52. The Japanese automaker's U.S. sales were down almost 10% last year, with the Nissan brand down 8.7% and its Infiniti luxury brand off 21.1%. Many analysts expect total U.S. auto sales to drop this year.