A sharp uptick in demand for light trucks, especially pickups, is leading to production capacity increases for some OEMs, especially Ford Motor Co.

Ford said it will produce nearly 40,000 additional vehicles between June and August by cutting its “summer shutdown” at 13 of its North American factories, including six assembly plants, from the traditional two weeks down to one.

“We are working most of our North American plants at maximum capacity, and we are adding production shifts in three of our assembly plants this month alone,” noted Jim Tetreault, Ford’s vice president for North American manufacturing. “Requiring more capacity from our plants is a good problem to have.” The Dearborn, MI, and Louisville, KY, truck plants, where the popular F-150 pickups are built, are among the Ford factories taking just one week of summer shutdown.

Ford noted its F-Series pickup truck line overall witnessed a 4% increase in sales in April with 47,453 trucks sold, the company’s best April showing in five years. Year to date, F-Series sales are up 11% through April with 191,280 vehicles sold; truck sales overall are up 7.6% through April at 260,644 units. Other truck makers are also experiencing strong demand for light-truck models. Chrysler’s Ram Truck division, for one, said pickup truck sales were up 19% in April versus the same month in 2011, with both Ram light- and heavy-duty pickup trucks posting sales increases. Light-duty models sporting crew cabs posted the largest percentage increase.

Overall, Chrysler said sales of Ram pickups increased 26% to 88,590 units year to date, with total truck sales up 21% to 369,243 units year to date. General Motors noted sales of its GMC Sierra pickup increased 20% in April this year versus the same month in 2011, one factor that convinced the OEM to increase its full-year light vehicle sales forecast to 14 million units.

“We expect gradual improvement in the economy going forward,” said Don Johnson, GM’s vice president of U.S. sales operations. “Over time, strength in the manufacturing sector and strong retail sales will lead to more job creation. That will help more consumers put the recession behind them, gain even more confidence, and drive vehicle sales higher for the industry.