The "corporate average fuel economy" standards will vary by company depending on the mix of models they sell. The requirements will be lower for companies such as General Motors, Ford and Chrysler, which offer more pickup trucks.
The Government can lower the standards if people suddenly start buying less-efficient vehicles, although few expect that to happen.
The Administration says the changes will save families up to US$7400 ($9230) on fuel over the life of a vehicle.
Environmental Protection Agency administrator Lisa Jackson says the standards also are the biggest step the US Government has ever taken toward cutting greenhouse gas emissions.
Tailpipe emissions from cars and light trucks will be halved by 2025.
President Barack Obama said the new fuel standards were the single most important step his Administration had taken to reduce American dependence on foreign oil.
But Republican presidential nominee Mitt Romney has opposed the standards, saying any savings at the pump would be wiped out by the rising cost of cars and trucks.
Already, carmakers have committed to an average of 6.6L/100km by 2016 under a deal reached with the Government three years ago.
The rules don't mean each new car or truck will get 4.3L/100km. The average vehicle will get closer to 5.8L/100km. Carmakers will be able to sell pickup trucks and less efficient vehicles as long as that's offset by smaller vehicles that already can get up to 5.8L/100km.
Manufacturers can reduce the mileage they're required to get with credits for selling natural gas and electric vehicles, using less polluting air conditioning fluid and adding stop-start circuits that temporarily shut off the engine at stop lights.
At showrooms, dealers are likely to 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.
Carmakers have already been adding technology to boost the efficiency of petrol engines, mainly because people want to spend less at the pump.
Fuel economy is the top factor Americans consider when buying a car, according to the research firm J.D. Power and Associates.
Fuel efficiency has been rising for the past five years because government regulations and high fuel prices have encouraged smaller vehicles and engines.
The Administration estimates that the new rules, combined with those that took effect last year, will increase the price of a new car about US$2800 by 2025.
But the Government says the net savings from the requirements will still be US$3500 to US$5000 because people will spend less on petrol.
The Administration also predicts that the new regulations will cost the car industry about $135 billion from 2017 to 2025.
The new rules were adopted after an agreement between the Administration and 13 manufacturers last year. That's a change from the past, when carmakers fought the regulations, saying they cost too much.
Industry leaders repeatedly told the Obama Administration that they wanted one nationwide fuel standard, fearing separate mileage standards from California and other states.
"They wanted certainty so that as they invest in the future they will know what rules they are playing by," the EPA's Jackson said.
Fuel economy standards were first imposed on US carmakers in the 1970s.
The aim was to make cars more efficient and reduce America's dependence on foreign oil at a time when the Arab oil embargo was creating shortages.
The requirements, which can be imposed without congressional approval, will be reviewed in 2018 and could be reduced if the technology isn't available to meet the standards.
The rules are tough, but General Motors, the largest US car company, will introduce features to comply, spokesman Greg Martin said.
"Consumers want higher fuel efficiency in their cars and trucks, and GM is going to give it to them."
-AP
