Detroit, May 1 (BridgeNews) - Sales at the United States' three biggest auto makers fell by double-digit percentages in April, but the companies are still fairly optimistic about the outlook for the full year. General Motors Corp. (GM), the world's largest car seller, said its U.S. vehicle sales fell 16% from a year earlier to 359,499 units. Sales at Ford (F) fell 14.5% to 318,812, while the Chrysler Group's (DCX) sales fell 18% to 187,119 units.