General Motors Corp., still struggling to meet the current demand for small fuel-efficient cars, is going to keep its only U.S. factory for compact cars working overtime for the rest of 2008, according to the carmaker.

GM doesn’t have a big enough supply of Chevrolet Cobalt cars, even though they added a third shift over the summer in the assembly plant in Lordstown, Ohio.

Sales for the Chevy Cobalt dropped 17% in September from last year at the same time, which GM claims was due to short supply. Sales for the Cobalt this year have increased 6%, while overall sales for GM have dropped 18% in a U.S. car market that is depressed.

The carmaker has been racing to makeover the manufacturing operations in the U.S. geared toward making large SUVs and trucks, which Americans have ultimately lost interest in. Consumers are feeling the pinch because of the poor economy and high fuel prices and are demanding small fuel-efficient cars with good new car warranties.