Toyota says its new Prius will have an improved battery and achieve better mileage. It goes on sale in the U.S. early next year.

TOKYO — Toyota released details for its fourth-generation Prius today, promising that improvements in the battery, engine, wind resistance and weight mean better mileage for the world’s top-selling hybrid car.

It gets up to 40 kilometers a liter under Japanese tests, which translates to more than 93 miles per gallon, a 20 percent improvement from the 2015 model. In the U.S., it gets 55 mpg in combined city and highway driving, about 10 percent better.

The Prius, which debuted in 1997, goes on sale in Japan in December and rolls out in the U.S. early next year. A price was not announced, but it’s expected to stay in the same range. The Prius now starts at about $24,000 in the U.S.

A hybrid switches back and forth between a gas engine and an electric motor to deliver a super-efficient drive, but is increasingly being challenged by even more futuristic technologies, such as the purely electric car and the fuel cell, which are both zero emissions.

Toyota said the efficiency of the Prius engine was beefed up 40 percent with reduced friction and improved combustion, while newly developed batteries are 10 percent smaller yet have 28 percent better performance. The battery, which had been in the back luggage area, now fits under the back seat, meaning more luggage space.

Toyota also made a point to deliver a smoother feel in driving, including four-wheel drive, as well as just-developed safety technology, including connecting with traffic lights at street corners.

The 2016 Prius was shown at the Las Vegas auto show, but Toyota didn’t release details then. Toyota has sold about 4 million Prius vehicles globally, but its popularity is mostly limited to the U.S. and Japan. The drop in oil prices also works as a minus.