General Motors Corp.'s (GM) shares touched their lowest price in more than 20 years Thursday, in keeping with the rough road the world's largest automaker has traveled in 2005 and uncertainty about the future.

An expected weak end to GM's U.S. sales year, a possible change in pension accounting that could erase most shareholder equity and investors taking their losses in the stock to offset gains in other investments depressed GM shares Thursday, analysts said.

GM shares slipped as low as $18.33 and were still off 19 cents at $18.42 on the New York Stock Exchange.

"We have today and tomorrow as the last trading sessions of the year. With the stock being down so far, you would be looking at a little bit of tax selling to harvest those capital losses to offset gains in other areas," Argus Research analyst Kevin Tynan said.

"I wouldn't say it's any one issue or any one news item. It's a combination of what's being going on ... with the timing of having two days left," he added, also referring to an expected weak U.S. sales report for December next week and the possible hit from a change in U.S. pension accounting rules.

Other analysts had similar reactions to the share decline.

"It's tax loss selling. I haven't heard of anything else," said Tim Ghriskey, chief investment officer of Solaris Asset Management, who does not own any auto stocks but follows them closely.

So far this year, GM shares have fallen almost 54 percent, making the stock one of the worst performers in the Standard & Poor's 500 index.

Reflecting the tough year for the U.S. auto sector, the list of 2005 underperformers also includes Ford Motor Co. (F) and Dana Corp. . On Thursday, Ford was off 9 cents, or 1.1 percent, at $7.75, while Dana was up 5 cents at $7.04.

GM's rough year is also expected to be reflected in weak U.S. auto sales for December, said analysts, some of whom expect the automaker to post monthly declines of 8 percent or more from last year.

Another reason for concern is GM's possible exposure if next year the Financial Accounting Standards Board (FASB), which sets U.S. accounting rules, requires companies to show a liability from pensions instead of a net asset. Such a move could wipe out GM's shareholder equity, the Wall Street Journal reported Thursday.

FASB voted earlier this year to undertake a project to make companies' pension information "more useful and transparent." The first phase of the project would put full pension liabilities on corporate balance sheets, possibly by the end of next year.

The proposed changes would likely affect auto and steel companies' shareholder equity the most, said Dan Noll, Director of Accounting Standards for the American Institute of Certified Public Accountants, but the changes, which would also require recognition of retiree health care benefits, could have a big effect on all companies' balance sheets.

"The information is already there in the financial statements and notes, but now moving it on the balance sheet, one has to wonder if investors will get more jittery," Noll said.