HOUSTON — Oil prices tumbled more than 7 percent on Tuesday, falling to their lowest levels in more than a year, after investors learned that Russia and the United States were pumping a lot more oil than had been expected.

The American benchmark fell below $47 a barrel for the first time in 15 months, capping a slide that has brought prices down by more than a third since early October.

Bountiful, cheap oil supplies are an unanticipated holiday bonus for American consumers. The average price of regular gasoline has fallen to $2.37 a gallon, according to the AAA motor club, 26 cents lower than a month ago. For most of the year gasoline prices were rising, but the recent decline in oil prices has driven gasoline down by about a nickel a gallon compared with a year ago.

Oil prices had stabilized earlier this month when the Organization of the Petroleum Exporting Countries and Russia agreed to slash production by 1.2 million barrels a day. But Russia announced on Monday that its output had increased to more than 11.4 million barrels a day, a record, putting in doubt its commitment to coordinate policies with Saudi Arabia and other oil producers.