"We knew when we saw crude oil prices drop last week that we'd break the $2 threshold pretty soon, but we didn't know if it would happen in South Carolina, Texas, Missouri or Oklahoma," said DeHaan, senior petroleum analyst for GasBuddy. "Today's national average, $2.74, now makes the current price we pay a whopping 51 cents per gallon less than what we paid a year ago."

Gasoline is sliding after OPEC decided last week not to cut production amid a global glut of oil that has already dragged international oil prices down by 37 percent in the past five months. Pump prices have fallen by almost a dollar since reaching this year's high on April 26.

Fifteen percent of the nation's gas stations are selling fuel below $2.50 a gallon, "and it may not be long before others join OnCue Express in that exclusive club that's below $2," said Gregg Laskoski, another senior petroleum analyst with GasBuddy.

An Oncue Express station in Oklahoma City was selling the motor fuel for $1.99 a gallon today, becoming the first one to drop below $2 in the U.S. since July 30, 2010.

Retail gasoline averaged $2.746 a gallon in the U.S. yesterday, data compiled by Florida-based motoring club AAA show. Stations will cut prices by another 15 to 20 cents a gallon as they catch up to the plunge in oil, Michael Green, a Washington-based spokesman for AAA, said by e-mail today.