"It's not going to be as much about volume," he said. "The battlefield will shift more toward market share." Besides G.M., other automakers reporting sales declines in March from the previous year included Ford Motor, Honda and Nissan. Ford, the second-largest domestic auto manufacturer, said its sales fell 3.5 percent to 234,000 vehicles, with its Ford brand and Lincoln luxury lineup both falling by similar amounts. The automaker continues to feel the pain of sagging F-150 pickup sales as it introduces its mostly aluminum-bodied version of the truck that for years has been the best-selling vehicle in the United States. In March, sales of F-series pickup trucks dropped 4.6 percent. Ford is trying to give Lincoln a boost at the New York auto show by unveiling the Continental, a huge luxury car aimed at buyers in the United States and China.