WASHINGTON — The Obama administration sought to gain momentum Tuesday for global talks on climate change by announcing details of its plan to require better gas mileage for cars and trucks and the first-ever U.S. rules on vehicle greenhouse gas emissions.

Transportation Secretary Ray LaHood and Environmental Protection Agency Administrator Lisa Jackson released the proposed regulations at the White House, following up on President Barack Obama's announcement in May that his new government would link emissions and fuel economy standards.

"This action will give our auto companies some long-overdue clarity, stability and predictability," Obama said Tuesday during a visit to a General Motors plant in Lordstown, Ohio.

It also could provide the Obama administration momentum on climate change in advance of a series of high-level talks on a new international agreement to curb heat-trapping gases and a speech on global warming by the president next week at a special U.N. summit.

The administration's lead climate negotiator just last week acknowledged that negotiations so far have failed to bridge the divide between developed and developing nations, saying that action was urgently needed on passage by the United States of legislation to limit greenhouse gases. The bill is delayed in the Senate by the health care debate, and the chances are growing dimmer that Congress will act before 180 nations gather in Copenhagen, Denmark, in early December to work on a new climate treaty.

In another prelude to Copenhagen, the Obama administration announced that the United States, Mexico and Canada would ask 195 nations that ratified a U.N. ozone treaty to enact mandatory reductions in hydrofluorocarbons, or HFCs, powerful greenhouse gases used as coolants in refrigerators and air conditioners.

The State Department said the proposal, if adopted, represents "a significant down payment" on efforts to broker a new climate change agreement this year and would help the United States achieve the president's goal of having reduced greenhouse gases by 80 percent come 2050.

Hydrofluorocarbons account for only about 2 percent of the globe's climate-warming gases, but their share is expected to grow to up to a third of all greenhouse gases by mid-century. That is because the 21-year-old ozone treaty, known as the Montreal Protocol, promoted their use to replace chlorofluorocarbons, which harm the ozone layer.

The new fuel economy standards, in the meantime, could serve as a placeholder, a concrete step by the Obama administration to reduce greenhouse gas emissions.

The agencies must complete the proposal by March 30 to give automakers enough planning time for the regulations to take effect in the 2012 model year.

"This marks a significant advance in our work to protect health and the environment and move our nation to a sustainable economy in the future," said Jackson, whose EPA is the main U.S. agency dealing with protecting the climate.

The new standards specify that the auto industry's fleet of new vehicles average 35.5 miles per gallon (15.1 kilometers per liter) by 2016.

The proposal will cover vehicle model years 2012 through 2016, allowing auto companies to comply at once with all federal requirements as well as standards pushed by California and about a dozen other states.

The administration estimated the requirements would cost up to $1,300 per new vehicle by 2016. It would take just three years to pay off that investment, the government estimates, and the standards would save owners more than $3,000 over the life of their vehicle through better gas mileage.

Jackson said the new standards will have the effect of taking 42 million cars off U.S. highways.

The proposal is expected to increase vehicle fuel efficiency by about 5 percent annually and reduce greenhouse gas emissions by nearly 950 million metric tons. The plan also would conserve 1.8 billion barrels of oil, Jackson said.

Administration officials noted that the new standards are four years ahead of a 2007 law that would have required the auto industry to meet a 35 mpg (14.9 kpl) average in 2020.

The proposed rules are expected to provide automakers some flexibility in meeting the requirements in exchange for building advanced vehicles. Some luxury automakers and foreign manufacturers who sell a limited number of vehicles in the United States could meet a less-stringent standard in the early years of the regulations.

LaHood said the administration had paid out $2.5 billion as of Tuesday for the Cash-for-Clunkers program under which auto dealers sold more than 700,000 vehicles in 30 days. LaHood said the remainder of federal payments to dealers under the program would be paid by the end of the month, taking government payments to a total of $3 billion.

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