Shares in General Motors, Ford and Fiat Chrysler — the three biggest US carmakers — all fell on Friday after weaker than expected domestic sales increases reignited debate about a possible downturn in the industry. GM said it sold 0.9 per cent more vehicles in March than in the same month last year. Its retail sales — to private consumers — were up 7 per cent year-on-year, but fleet sales declined following a decision to restrict sales to car rental companies, which were down 36 per cent. Meanwhile, Ford, the market number two, and Fiat Chrysler, the number four, announced 8 per cent sales increases. Ford’s sales were led by a 13 percent surge in sales of its Sports Utility Vehicles, which have enjoyed a boost from falling fuel prices. Fiat Chrysler’s sales benefited from the latest in a string of strong performances from its Jeep SUV brand, whose sales were up 15 per cent. However, in lunchtime trading, Ford’s shares were down 3 per cent at $13.10, GM’s down 3.1 per cent at $30.46 and Fiat Chrysler’s down 4.7 percent at $7.68. Michelle Krebs, an analyst at Autotrader.Com, said the negative reaction to the sales figures reflected a nervousness over the sustainability of last year’s record 17.5m US auto sales. GM had previously said it expects sales to “plateau” rather than fall this year, but some analysts predict that growing economic concerns and rising interest rates will start pushing sales down.