While the regulations have been in development for more than a year, the White House’s decision to make them final on the first full day of the Republican National Convention seemed intended to highlight one of President Obama’s proudest accomplishments at a time when Mr. Romney has laid out a different energy and environmental agenda.

The administration called the new rules “historic,” and estimated that Americans would reduce their oil consumption by 12 billion barrels over the course of the program. “These fuel standards represent the single most important step we’ve ever taken to reduce our dependence on foreign oil,” Mr. Obama said in a statement.

But the Romney campaign has criticized the new rules as “extreme” and said the standards would limit the choices when consumers shop for a new car. “The president tells voters that his regulations will save them thousands of dollars at the pump, but always forgets to mention that the savings will be wiped out by having to pay thousands of dollars more upfront for unproven technology that they may not even want,” said Andrea Saul, a spokeswoman for the Romney campaign.

The transportation secretary, Ray LaHood, said the standards would save Americans $1.7 trillion in fuel costs, resulting in an average savings of more than $8,000 a vehicle by 2025.

The fuel savings, he said, would easily exceed the estimated $2,000 to $3,000 that the more efficient vehicles would cost consumers to buy.