But Michael Robinet, managing director of IHS Automotive, which tracks production, said many U.S. auto plants are running almost around-the-clock to meet demand. By the end of the year, more than half the vehicles made in North America will come from plants running the maximum three shifts, Robinet said."We're really bumping up against the edge," he said. "So it really is brick-and-mortar time." Ford is already producing as many Fusion midsize sedans as it can sell, said Erich Merkle, the company's top U.S. sales analyst. Supplies will be low until this fall. That's when Ford adds a new shift with 1,200 workers at the Michigan plant that will build the Fusion. "We're going to need more capacity," said Merkle. Ford plans to increase production in North America by 9 percent in the second quarter. But after going through a severe downsizing during the recession, companies are also wary of adding factory space. They fear the cost of having too much if the economy heads south. GM said Tuesday that it has no plans to increase production because its factories can handle the expected slow-and-steady growth. Ford and GM each reported sales gains of 6 percent for March. GM reported big increases for Buick and Cadillac, which both have new small cars. Ford's Fusion sedan and Escape SUV both reported record monthly sales. Still, Ford's Lincoln luxury brand was down 23 percent. But the arrival of the MKZ sedan at dealerships this month should help turn sales around, the company said. Toyota sales rose 1 percent. Sales of the new Toyota Avalon and Lexus ES sedans doubled over last March, making up for declines for the Camry, RAV4 and most of Scion's lineup. Other automakers reporting Tuesday: â¿¿ Chrysler's sales rose 5 percent thanks to a 25 percent increase for the Ram pickup and big gains for the Dodge Avenger and Challenger sedans.