Late last month, the Obama administration and 13 automakers announced plans to double the U.S. corporate average fleet economy (CAFE) standard from 27.3 miles per gallon in 2011 to 54.5 mpg by 2025. The new goal will require gradual improvements, beginning in 2017, in the efficiency of passenger cars and light trucks, along with reductions in their greenhouse-gas emissions.

Automotive fuel economy regulations in the United States have rapidly grown more stringent since 2007, when President Bush signed legislation calling for new vehicle fleets to average 35.5 mpg by 2020. (For the previous two decades, the standard held steady at around 25 mpg.) In 2009, the Obama administration moved the target date for 35.5 mpg up to 2016.

Though automakers resisted past increases in the CAFE standard, this time they’ve been much more supportive.  It helps that the administration set less stringent standards for pickup trucks than for passenger cars. And a midterm review is scheduled, to reëvaluate the feasibility of the rules for 2022 to 2025 before finalizing them. Perhaps most important, some experts say, is that there’s enough room for improvements to power trains and auto-body designs—and enough time to make those improvements—that automakers can satisfy the new rules without counting on major technological breakthroughs.

The standard will, in fact, be less stringent than it may seem. Automobile testing takes place in a laboratory, not on the road, so the results need not reflect real-world fuel economy.  The real-world performance will be more like 40 mpg. The actual number will become clearer in September, when the Environmental Protection Agency and the National Highway Transportation Safety Administration release the official details of the plan.

However, it remains a “very challenging target,” says John Heywood, a professor of mechanical engineering at MIT and former director of MIT’s Sloan Automotive Laboratory. Reaching it, he says, will require automakers to make significant changes to the technologies they use and the mix of models they offer consumers. Heywood estimates that up to two-thirds of the required improvement could come through changes to conventional gasoline and diesel engines, increases in the efficiency of hybrids, and greater market penetration of hybrids.

“Our expectations are that both [hybrids and conventional vehicles] will improve in a proportional way,” Heywood says. He adds, “Hybrid technology might progress faster because it is newer.”

The rest of the change needed, Heywood estimates, could be accomplished by bringing down average vehicle weight. Automakers might both redesign individual models—for instance, replacing heavy steel with lighter materials—and increase the proportion of smaller vehicles they offer.

Hitting the new CAFE mark is likely to depend on the continued evolution of certain technologies, such as variable valve-timing systems and turbocharged gasoline direct-injection engines, but no fundamental breakthroughs will be required, says John DeCicco, who researches transportation energy use and greenhouse gas emissions at the University of Michigan.  “All the technologies we are talking about are essentially already in the marketplace in one form or another,” says DeCicco. “This is engineering we understand well.”

DeCicco’s view is that plug-in hybrids and pure plug-in electrics—for which breakthroughs are needed to bring down battery cost—will probably still be a niche market in 2025. But that won’t affect automakers’ ability to meet the CAFE standard, he argues. “You just don’t really need them.”

Ultimately, reaching the goal set forth in the CAFE standard may be realistic from a technological standpoint, but it will require reversing a trend of the past few decades: steady increases in vehicle weight and fuel consumption. “The amount of change required as we go 15 years into the future is bigger than people are mentally allowing for because things have been getting worse,” says Heywood. “It’s not only that it needs to get better. We need a shift from a getting-worse trajectory to a getting-better trajectory.”