DETROIT — General Motors announced yesterday plans to shrink pensions for salaried workers, a move that will save the automaker $420 million in 2007.

The changes will move newer salaried staff to a defined-contribution plan, which is more dependent on employee contributions. That follows a national trend of companies moving away from traditional pensions, which guarantee a monthly payout for life but cost employers more.

GM, which said last month that it would reduce or eliminate white-collar pensions, said salaried workers hired before Jan. 1, 2001, will hold on to the promise of a traditional pension. But GM will slash how it calculates future benefits.

Salaried employees hired after Jan. 1, 2001, will be shifted entirely to a defined-contribution plan. Essentially, they will save for retirement through a 401(k) plan. As part of that, GM will contribute an amount equal to 4 percent of annual base salary to the employee's 401(k).

More than 42,000 workers will be affected by the changes, expected to reduce GM's pension liability by $1.6 billion annually. At the end of 2005, its pension liability was $10.82 billion, according to its most recent financial statements.

"These changes will reduce financial risks and future costs for GM, while protecting current retirees' and employees' earned pension benefits and providing competitive and fair retirement benefits going forward," CEO Rick Wagoner said.

Wagoner's own retirement plan will be changed. He is among nearly 2,000 top executives who will move to the U.S. salaried pension plan and away from a more lucrative supplemental program.

Pensions have been disappearing throughout the country, even at companies with healthy balance sheets. The American Benefits Council says the number of pension plans is half what it was a decade ago.

Workers like defined-benefit plans because they draw a set amount of money in retirement. In defined-contribution plans, money contributed a 401(k) grows over time. But there is no guarantee what it will be worth in retirement.

GM's pensions cover 600,000 people, including white- and blue-collar workers and retirees. Analysts at Fitch Ratings expect GM to begin moving United Auto Workers members away from defined-benefit plans when its contract with the union reopens in September 2007.

"The environment could be conducive to negotiating changes to the traditional defined-benefit structure," the agency said in a recent research note, citing financial stress in the industry and at GM, the potential for pension reform legislation in Congress and the continuing trend away from defined-benefit plans.