Lighter cars with smaller engines could be on the way, along with more electric cars and hybrids

The Associated Press is reporting that the average gas mileage of new cars and trucks will have to nearly double by 2025 under regulations that were finalized today by the Obama administration.

The new rules will require the fleet of new cars and trucks to average 54.5 miles per gallon in 13 years, up from 28.6 mpg at the end of last year.

The regulations will bring dramatic changes to the cars and trucks in U.S. showrooms and drive automakers to introduce new technology to make vehicles cleaner and more efficient.

The administration says the changes will save families more than $1.7 trillion in fuel costs and bring an average savings of $8,000 over the lifetime of a new vehicle sold in 2025. The standards also are the biggest step the U.S. government has ever taken toward cutting greenhouse gas emissions, Environmental Protection Agency Administrator Lisa Jackson said. Tailpipe emissions from cars and light trucks will be halved by 2025, the government said.

Republican presidential candidate Mitt Romney has opposed the standards, and his campaign on Tuesday called them extreme and said they would drive up the price of new cars. Any savings at the pump would be wiped out by rising costs of cars, the campaign said.

By 2025, some bigger models may disappear, and dealers could offer more efficient gas-electric hybrids, natural gas vehicles and electric cars. There also will be smaller motors, lighter bodies and more devices to save fuel, such as circuits that temporarily shut off engines at traffic lights. The changes will raise new car prices, but the government says that will be more than offset by savings at the pump.

The gas mileage requirements will be phased in gradually and get tougher starting in 2017. They build on a 2009 deal between the Obama administration and automakers that committed cars and trucks to average 35.5 mpg by model year 2016.