President Barack Obama won praise from the auto industry last week when he outlined plans to come up with fuel economy standards for 2017-25 model cars and light trucks plus 2014-18 model medium- and heavy-duty trucks and buses.
Automakers have pushed for a long-range program and said the process would continue the path to a single national program for fuel economy and greenhouse gas emissions.
The companies have argued that they need long lead times to develop cost-effective technology. Similarly, energy providers have said they require time to expand infrastructure and availability of low-carbon fuels.
"Introducing new technologies and fuels to consumers takes time to get up to speed," said Dave McCurdy, president of the Alliance of Automobile Manufacturers, which represents 11 domestic and foreign automakers.
Automakers also lauded Obama for continuing a plan begun a year ago to create a single national program.
Previously, they had complained about the difficulties in complying with a regulatory patchwork that had the National Highway Traffic Safety Administration, the EPA and California and 13 other states applying their own standards.
Obama's long-term plan builds on rules set last month for 2012-16 light vehicles. They require average fleet fuel efficiency of 35.5 mpg by 2016.
Last week the president also said he would begin development of an advanced infrastructure for plug-in hybrids and electric vehicles.
Obama did not specify any fuel efficiency goals. Instead, he set up a planning process involving the federal agencies, the industry, the states and environmental groups.
The administration is to announce by September its plan to set light-vehicle standards. Regulators were told to issue a final rule for medium and heavy trucks by July 2011. It will be the first standard of its kind in that sector.
As a presidential candidate, Obama said the United States could achieve a 50 mpg average for light vehicles by 2027.
A spokesman for the Union of Concerned Scientists said that goal could be accomplished without much additional cost for automakers.
"You can get to that level and beyond with technology on the road today, and you can well exceed it by adding electric vehicles and plug-in hybrids," said Jim Kliesch, a senior engineer for the nonprofit group.
But Dave Cole, chairman of the Center for Automotive Research, cautioned that aggressive regulation could price vehicles out of consumers' reach.
"Regulation must be in line with technological and economic reality," he said. "If not, then consumers won't accept it."
Jesse Snyder contributed to this report