Automakers would have until 2020 to raise the fuel efficiency of their cars and trucks to an average of 35 miles per gallon under a key U.S. Senate proposal unveiled Friday, but the auto industry's largest trade group said the plan was unacceptable.

The amendment from Sens. Daniel Inouye, D-Hawaii, and Ted Stevens, R-Alaska, will be the starting point when the Senate Commerce, Science and Transportation Committee meets Tuesday to consider passing a fuel economy increase proposal and sending it to the full Senate.

The auto industry has opposed several plans that would require 4% annual increases in fuel economy; many of those bills also had set overall standards as high as 40 m.p.g. by 2017. While President George W. Bush had set a 4% goal of his own, his plan would leave the final call on higher standards to federal regulators, as automakers prefer.

Under the Inouye-Stevens proposal, cars and trucks would have to meet an interim goal of 28.5 m.p.g. by 2015, and face a 4% annual increase after 2020, unless federal regulators found that such increases weren't cost effective or could hurt safety. Medium and heavy-duty trucks would face a 4% annual increase starting in 2011, and regulators could set rules for cars based on size, as they have done for trucks.

The plan also would end by 2009 the credit U.S. automakers get under federal fuel economy standards for selling vehicles capable of burning E85, a blend of 85% ethanol and 15% gasoline. That credit long has been opposed by environmental groups, which contend the credits actually increase gas consumption because few drivers use E85 regularly.

While the industry averaged 25.4 m.p.g. for the 2006 model year, it included credits to General Motors Corp., Ford Motor Co. and the Chrysler Group for millions of flex-fuel vehicles, and the three have pledged to make half their new vehicles flex-fuel capable by 2012. Removing those credits would mean automakers have to make even greater improvements in efficiency to hit the 35-m.p.g. target.

A spokeswoman for the Alliance of Automobile Manufacturers, the industry's main trade group, said the new proposal was a mash-up of poor ideas from previous bills, noting that the 35-m.p.g. goal equaled a roughly 4%-a-year efficiency increase between now and 2020.

"It's unattainable up to 2020, and unattainable thereafter," said AAM spokeswoman Gloria Bergquist. Removing the flex-fuel credit is "really at cross purposes with the entire goal of reducing gasoline use."

But David Friedman, an auto industry researcher with the Union of Concerned Scientists, said the interim goals and leeway given to federal regulators made the bill a weaker proposal than necessary.

"It's a mix of good and bad news," he said. "It's pretty clear that the leaders and many important members in Congress are determined to get a fuel economy bill out of the Commerce Committee."