– Automakers have steadily improved the fuel economy of new vehicles, pushed by federal regulations. While some companies have introduced hybrid and electric cars, most of the recent gains have come from improving traditional gasoline-powered vehicles.

The Trump administration is proposing to weaken those Obama-era vehicle rules after 2021. A big question is whether automakers can meet increasingly strict standards in the years ahead. Here's a look at how automakers have improved the fuel economy of some of their most popular models so far — and what the future might hold.

Toyota has increased fuel economy in the Camry through a multitude of engine improvements. That can add to the upfront cost of the vehicle, although drivers save on fuel costs over time.

Other automakers have adopted different strategies. Chevrolet has increased the fuel economy of its Malibu sedan by switching to a smaller engine that uses less fuel but still maintains power by using turbochargers. The Malibu also has a start-stop system that shuts the engine off when the car comes to a stop.

And analysts say that automakers haven't yet hit the limits of improvements in internal combustion engines. Mazda has announced a new engine technology for 2019 that it claims can improve fuel efficiency even more by more precisely controlling the timing of fuel ignition.

Because of how the current fuel economy rules are structured, SUVs face less stringent standards than smaller cars do. So, as Americans have shifted toward larger vehicles in recent years, that has temporarily slowed the overall pace of fuel economy gains.

But SUVs still face tighter requirements each year. Subaru has improved the fuel economy of its Outback by redesigning the transmission. Having more gears can allow an engine to operate more efficiently at different speeds, and the Outback's continuously variable transmission essentially allows for a nearly infinite set of gears, helping to improve fuel economy.

Automakers are still trying to perfect these newer transmission systems, which, if implemented poorly, can lead to jarring driving experiences or higher maintenance costs. But analysts expect that as companies like Subaru and Honda fine-tune the technology, it will become more widespread to help meet stringent fuel economy standards.

The Ford F-150 pickup has long been the best-selling vehicle in the United States. As vehicle standards have risen, Ford has improved the truck's fuel efficiency by removing 700 pounds, switching from steel to high-strength aluminum. (The lighter F-150 has still performed well in crash tests.)

The company also developed smaller engines that use less fuel while maintaining power. Ford's most popular F-150 engine today now produces slightly more horsepower than the most powerful engine available in 2010. And fuel economy has gone up by 40 percent.

The F-150 still consumes far more gasoline per mile than smaller cars like the Camry or the Malibu do. But improving the fuel economy of a gas-guzzling pickup truck by 1 mpg actually reduces overall gasoline use by more than eking out another 1 mpg from a highly efficient sedan, because of diminishing returns, said Gabriel Collins, an energy fellow at Rice University's Baker Institute for Public Policy.

"Large heavy vehicles are really your low-hanging fruit if you're looking to rein in fuel use and emissions," Collins said.

If fuel economy standards continue to rise, automakers may need to produce more hybrids and electric cars to meet future rules.

Automakers have recently rushed to announce new electric models, and Ford is planning a hybrid F-150 by 2020. But they have also expressed concern about consumer willingness to buy these vehicles in the short term. Last year, plug-in vehicles made up just 1 percent of new sales.

The Alliance of Automobile Manufacturers, the main automaker lobbying group, has asked the Trump administration to relax the vehicle standards between 2022 and 2025, arguing that current standards would require roughly one-third of new vehicles sold in 2025 to be full hybrids. "Consumer adoption of advanced technology vehicles has not lived up to expectations," Mitch Bainwol, president of the alliance, told Congress last year.

But other groups argue that automakers can readily meet the Obama-era standards without needing to shift heavily into electric vehicles. "There's no question in my mind about that," said John German, a senior fellow at the International Council on Clean Transportation, pointing to the latest Camry, which already meets 2022 standards.

The administration may go even further and freeze the standards entirely at model year 2021 levels, according to an early draft of the proposed rules. If that proposal is finalized, automakers will no longer be required to invest in new fuel-saving technologies after the next few years.