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NEW YORK (CNN/Money) - General Motors' pension pain could soon spread to hundreds of other companies, even those offering relatively healthy traditional pension plans.

The numbers involved are huge: the federal agency that insures traditional pensions estimates that companies with plans that are underfunded by at least $50 million collectively need another $354 billion to make good on their promises to employees when they retire. (See correction).

GM (down $1.98 to $27.19, Research) stock tumbled Thursday after it said the Securities and Exchange Commission was investigating how it accounts for pensions and retiree health care coverage, as well as other parts of the automakers' accounting. (Full story).

The world's largest automaker says it's one of those companies with healthy pension plans, estimating that at the end of last year its plans had $1.5 billion more than needed to pay promised benefits, despite the company's steep recent losses.

Still, investors seem worried that the automaker might not be in as good a position as it claims. And experts say as more attention is given to the arcane world of accounting for benefits promised to retirees, other companies could also be hit by those doubts.

"It seems to be an extreme overreaction," said Mark Vitner, chief economist for Wachovia Securities, about the hit to GM stock, which pressured the broader market Thursday. (Full story).

"But pension accounting is awfully complicated and it's an awfully big company, so it's not surprising the markets would get a little spooked by it."

Part of the problem is that the estimates by GM and other companies about the strength of their plans is based on a set of assumptions about things that can't be predicted -- future interest rates and rates of returns on assets going forward, for example, as well as the life expectancy of employees and retirees, and when current employees will retire and start drawing benefits.

Those are assumptions that typical accounting never has to take into consideration.

Experts say even without any changes in the law, outside accounting firms and regulators are likely to get more conservative about those assumptions going forward, which will change the estimates of how pension funds will fare even without any changes in the plan assets or promised benefits.

Congress is looking at a number of options meant to make employers shore up traditional pension plans.

Already, some estimates using different assumptions show that the GM plans won't have enough funds to pay promised benefits, a status known as underfunded.

The company's current junk bond status is one of the factors that could change assumptions and accounting rules.

The biggest underfunding estimate would come from a worst-case scenario used by the Pension Benefit Guaranty Corp., the federal agency that backs pension plans in the private sector, which assumes a company will terminate the plans rather than continue to make contributions going forward.

"Other people can make certain assumptions that could change that (funding) number," GM spokesman Jerry Dubrowski said. "A minor change to one assumption can have a change in the funding status. It doesn't mean their number is any more or less correct. We believe our number is appropriate."

But those advocating rule changes say they're needed to make retirees' benefits more secure, or to protect taxpayers from having to pay for a savings and loan-type of bailout of the nation's pension plans that some people now fear.

Congress and regulators are looking into what is seen as a growing problem of underfunded plans. According to the PBGC, the plans with the biggest shortfalls are underfunded by an estimated $353.7 billion.

For all U.S. companies that offer traditional pension the figure is much higher, about $450 billion, an agency spokesman said.

Experts in the field say that both the critics and proponents of the proposed rule changes have good points.

"There's truth to both sides of the story," said Don Fuerst, worldwide partner at Mercer Human Resource Consulting. "All companies say 'That (plan failure) isn't going to happen to us.' But it's going to happen to some of them. Still it's not going to happen to everybody, so the PBGC numbers are a worst-case scenario."

Fuerst and other experts say they worry that tougher pension rules could mean even more companies will stop offering traditional pension plans, moving toward plans that place investment risk on the employees and retirees, rather than the companies and pension plans.

"We want to help companies that have an underfunded plan improve their funding and stay healthy, and not to give them reason to terminate a plan they otherwise want to keep," said James Klein, president of the American Benefits Council, an advocacy group for major employers on health and retirement issues.

Some of the companies offering plans argue that the proposed changes will make a bad situation worse, forcing companies facing other financial problems to make greater contributions when they are most strapped for cash.

"The best way to protect a pension plan is make sure the companies have the financial wherewithal to continue to operate," said GM's Dubrowski.

(Correction: An earlier version of this story didn't specify that the $354 billion pension shortfall was for companies with plans underfunded by at least $50 million. Back to story).

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