GM’s Pension Fund Shortfall Ballooning : Autos: With the ranks of the company’s retirees growing, the gap could hit $11.4 billion by the end of the year.

At a time when General Motors Corp. is herding thousands of salaried employees into early retirement and hoping to do the same with its hourly workers, its pension fund shortfall could swell to $11.4 billion by the end of this year, the company said in a filing Monday with the Securities and Exchange Commission.

The gap between what GM owes and how much it has paid into the fund has grown by $2.8 billion since the end of 1991. Closing the gap could put an added strain on GM’s cash reserves as the company struggles to return to profitability.

Also in the SEC filing late Monday, GM said that it will take a one-time $300-million charge against fourth-quarter earnings as it seeks a partner to “acquire a significant portion” of National Car Rental Systems. GM owns 81.5% of National.

GM said the charge will be partially offset by a one-time fourth-quarter gain of $225 million from the sale of its interest in a joint venture with Korean auto maker Daewoo and the sale of its Allison Transmission subsidiary.

At a news conference last week, GM Executive Vice President William E. Hoglund indicated that the pension fund shortfall is the result of two erroneous assumptions: that the funds would earn more and that retirees would die sooner.

GM is also grappling with a more universal problem for older U.S. industrial corporations--its work force is shrinking as the ranks of its retirees expand. The trend is expected to accelerate in coming years at the world’s largest auto maker. GM has about 360,000 retirees, a number nearly equal to its 370,000 active U.S. workers.

“If GM keeps shrinking, which is likely, fewer active workers will be supporting an increasing number of retirees, and that could prove to be a significant cost burden,” said Harley Shaiken, a professor of work and technology at UC San Diego.

The Associated Press contributed to this story.