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The trend has automakers introducing lighter vehicles and embarking on demonstration projects designed to carve hundreds of kilograms off their most popular vehicles. Last month, for example, GM received major awards for its new Cadillac ATS sedan that weighs 3,315 pounds (1503.7 kilograms), making it one of the lightest vehicles in its class, thanks in large part to an all-aluminium hood, magnesium engine mounts, and other lightweight materials.

“Every automaker we talk to is talking about how they can do more ‘lightweighting,’ ” says Patrick Davis, vehicle technologies program manager at the U.S. Department of Energy’s Vehicle Technologies Office. “Using lightweight materials is a major way the automakers are planning to meet fuel economy standards through 2025.”

As they contemplate the new fuel standards, which require cars to have an average miles-per-gallon rating of 54.5, automakers cannot rely on consumers to buy more hybrid or electric cars, or smaller, more efficient models.

Reducing the weight of conventional cars offers a way to guarantee better fuel efficiency: every 10 percent reduction in weight provides a 6 to 7 percent improvement in fuel economy. Weight savings can lead to further fuel economy improvements by allowing automakers to use smaller, lighter engines and other components. Davis says the DOE’s hope is that all cars will be 35 percent lighter by 2025.

Along with aluminium and magnesium, carmakers are using more carbon fiber. The material is lighter than steel but still absorbs more energy, which can help make lightweight vehicles safe. But it can cost three times as much to make. Oak Ridge National Lab (ORNL), working with Dow and Ford, is developing new precursor materials that, along with improvements to manufacturing, could cut the cost of carbon fiber in half, says Raymond Boeman, a program director at ORNL.

After decades of piling on the pounds, all sorts of cars are becoming lighter. Mazda’s 2014 Mazda6 is 8 percent lighter than the previous model; the chassis is 16 percent lighter, and its bumpers, featuring a novel resin Mazda helped develop, are 20 percent lighter. Jaguar and Volvo both plan to introduce lightweight vehicle platforms—structures that can be used in a wide range of future models—making extensive use of lightweight materials such as boron steel and aluminum.

Automakers are also slimming down their heavy vehicles, which have some of the biggest potential for reducing fuel consumption. Ford has replaced some of the steel in one of its most popular vehicles, the F-150 pickup, with aluminium; and it has partnered with Dow and ORNL in a bid to cut the weight of this vehicle by 750 pounds (340 kilograms) by the end of the decade. The new Range Rover, which went on sale last month, displaced enough steel with aluminum to cut its weight by more than 400 kilograms. The trend has created a boom in the aluminium industry, which expects demand for aluminum in automobiles to grow by 25 percent per year for the next several years.

Even smaller cars aren’t being spared the axe. The back windows of the new Fiat 500L will be made of plastic, making them half as heavy as glass ones. Peugeot, in collaboration with the oil giant Total, is building a demonstration version of its small 208 that is 200 kilograms lighter thanks to composites and plastics. The lighter design could result in a car with half the carbon emissions of its predecessor.

Vehicle lightweighting will also be crucial for more advanced, fuel-efficient cars, such as electric vehicles. This year BMW will introduce the electric i3, which will feature carbon fiber components to cut weight, helping to offset the weight of the battery pack and extend its range. BMW has also teamed up with Toyota to reduce the weight of vehicles through a research program that includes plan to develop fuel cells and advanced batteries; the German carmaker is part of a consortium that’s building an electric car that will weigh only 400 kilograms without its battery.

Davis says that automakers have been increasing their use of lightweight materials for decades (see our story from 1997, “A Practical Road to Lightweight Cars”), but that the weight savings has largely gone toward allowing them to add new features such as airbags and infotainment systems. Now they plan to reduce overall weight and improve fuel consumption. But he says it will take some time for the changes to have an impact on average vehicle weight, as automakers introduce new models a few years apart, and as they wait for volume production and materials advances to bring costs down.

As automakers make their vehicles lighter, they’re also attending to safety concerns. In a recent presentation analyzing the potential impact of larger numbers of lower-weight vehicles, the Insurance Institute for Highway Safety noted that, historically, about twice as many people die in the lightest cars as in the heaviest ones. Better vehicle designs, including safety features that prevent accidents by assisting drivers, will be needed to keep drivers safe (see “Audi Shrinks the Autonomous Car” and “Will Automated Cars Save Fuel?”).