WASHINGTON — In 2025, when the auto industry would be required to meet a 54.5-mile-per-gallon fuel economy target that the Obama administration is about to announce, the vehicles Americans buy might not look very different from those on the road today.

But what will be under the hood is another story, industry and environmental experts say. Almost every vehicle sold in the United States likely will boast the kinds of advanced technology now confined to only the most fuel efficient.

Not every truck and SUV will have to get the higher mileage. The standard will be applied to the so-called fleet average, a complex calculation of the combined fuel efficiency of all the vehicles sold by a manufacturer in a model year. That means some gas-guzzlers can still be sold; they just have to be offset by a larger number of highly efficient cars.

President Barack Obama plans to make a speech in Washington on Friday to outline a plan for boosting fuel economy standards beginning in 2017 and reaching a fleetwide average of 54.5 miles per gallon by 2025.

The new requirements build on rules starting for model year 2012 that seek to push up fuel economy to 34.1 miles per gallon by 2016. The more efficient cars and trucks are also expected to drive down the greenhouse gas emissions linked to global warming.

Right now, the American fleet of passenger vehicles averages only about 27.8 miles per gallon.

Each carmaker will probably have a different standard applied to it by regulators — following a political concession made in 2007 to get the support of domestic automakers.