Other manufacturers such as Toyota, which makes the popular Prius hybrid, aim to make hybrid engines at least an option, if not standard, in all their models by 2010.
The government's new fuel economy standards won't fully kick in until 2011, Kammen noted.
"That's why it's such a ho-hum piece of legislation. It could do more to push the fleet [of all U.S. cars and trucks] so much faster," he said.
Joan Claybrook, president of the nonprofit Public Citizen and administrator of the National Highway Traffic Safety Administration from 1977 to 1981, said the new rules are insufficient given the technology available.
"Currently, technology exists to achieve 40 miles per gallon [17 kilometers per liter] fleetwide for cars and trucks," she said in a statement.
"We can and must do much better."
While the plan will include SUVs weighing 8,500 to 10,000 pounds (3,855 to 4,536 kilograms), it excludes pickup trucks in this weight class.
According to the Sierra Club, an environmental organization, 80 percent of the vehicles in that class are pickup trucks, not SUVs, and will thus be exempt from the new standards.
"Since the administration continues to exclude heavy pickup trucks and is only including a handful of the heaviest gas guzzlers, the oil savings that will result from including these vehicles are minimal," the group said in a statement released Wednesday.
"Using the administration's estimate of oil savings, today's rule will save less than two weeks' worth of oil consumption at current levels over the next four years," the group added.
At Wednesday's announcement, Mineta defended the rules as a step toward the goals of reducing U.S. dependence on foreign oil and protecting the environment.
"Saving fuel is as important to our national security and economic vitality as it is to preserving the environment," he said.
"President Bush understands that and is committed to encouraging the kind of measures that will reduce our reliance on foreign oil."
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