A looming slowdown in global auto demand will test U.S. automakers General Motors Co and Ford Motor Co as they try to funnel more cash to shareholders while still investing adequately in new vehicles and technology. At a conference on the sidelines of the Detroit auto show, the rivals outlined their strategies for an era of slower growth and rapid shifts in technology and consumer behavior. As of Wednesday, GM (GM.N) was winning the referendum among investors, with its shares up slightly, while Ford (F.N) shares sank about 5 percent.