DETROIT - The U.S. auto industry powered ahead in August, shrugging off gyrating stock markets to stay on pace for the best annual sales since 2001. Consumers surprised industry analysts who had expected a decline in August sales. Early returns on Tuesday indicated sales of about 17.5 million vehicles on an annualized basis, easily besting the forecast of 17.3 million vehicles, according to a Thomson Reuters poll of 47 economists. Roller-coaster stock markets appeared to have no major impact on auto purchases, which are an early indicator of consumer spending each month, particularly for large-ticket items. “All of the economic fundamentals that we look at, including job growth, disposable income and fuel prices, are in good shape and that should keep sales strong,” said Kurt McNeil, head of U.S. sales for General Motors Co. GM, the No. 1 automaker in the U.S. market, reported that sales dropped 0.7 percent, better than analysts’ estimates for a decline of as much as 5 percent. Ford Motor Co, the No. 2 U.S. automaker by vehicle sales, showed a gain of 5 percent, easily outdistancing expectations of less than a 1 percent rise from a year ago. Fiat Chrysler Automobiles NV posted a stronger-than-expected rise of 2 percent, boosted by Jeep SUV and Ram pickup truck sales. Analysts looked for a decline of FCA sales. The Italian-American automaker kept alive its streak of consecutive months of topping year-ago sales figures - which now stands at 65. August sales were expected to be down largely based on a quirk in the calendar that put results over the U.S. Labor Day holiday weekend into September sales. Although Labor Day is always in September, the U.S. auto industry usually counts the sales in August results. Ford said sales of its F-Series pickup trucks topped 70,000 for the first time this year - at 71,332, up 4.7 percent. Mark LeNeve, head of Ford’s U.S. sales, said results were strongest for its newest models. SUV and truck sales both rose about 12 percent while car sedan sales fell 7 percent. GM’s Chevrolet Silverado and GMC Sierra pickup trucks together outsold Ford’s F-Series, at more than 76,000. Silverado sales rose 11.7 percent and Sierra sales were up 7 percent.