Oil prices have already fallen to their lowest level in 6 ½ years, and gasoline prices at the pump may soon follow suit.

Oil futures CLV25, fell below $40 a barrel on the New York Mercantile Exchange Monday, their lowest level since February 2009. On Wednesday, futures dropped below $39.

“If oil remains at this level, it is practically guaranteed that average gas prices will fall below $2 per gallon by Christmas,” said Michael Green, a spokesman for AAA.

A drop to under $2 would be the lowest price in 6 1/2 years.

Gasoline prices still have some way to go, however. Even as 12 states have at least one gas station selling regular gasoline for under $2 a gallon, as of Monday, the average price in those states remains well above $2, according to AAA’s Daily Fuel Gauge Report.

See: Oil turns lower as gasoline futures drop nearly 5%.

As of Wednesday, the average retail price for a gallon of regular gas nationwide stood at $2.556.

That’s below Monday’s $2.595, which was the lowest average for late August since 2004, so most drivers are paying the least for this time of year in 11 years, said Green. But the “irony of the situation is that gas should be much lower today given the current cost of crude oil.”

Based on history, “average gas prices should be at or below $2 per gallon when oil is cheaper than $40 per barrel,” he said. But “there is an enormous disconnect between oil and gasoline prices due to ongoing refinery problems and record-high driving demand.”

Among the refinery problems was an unscheduled outage at BP PLC’s BP, -2.96% UK:BP Whiting, Ind., refinery, the largest in the Midwest. That sent prices climbing throughout the Midwest. BP confirmed a restart of the Whiting refinery late Tuesday following the shutdown that began on Aug. 8.

Read: Why gas prices are rising as oil falls to 6-year lows

Aside from this week, the last time West Texas Intermediate crude oil was under $40 was on Feb. 24, 2009. At that time, the national average price of gas was $1.90 a gallon, according to Green.

Only once before has the national average price of gas been higher than $2 when oil was under $40, based on AAA records going back to 2000, he said. That was for one week in early 2004, and after climbing to $2.04 for that period, gasoline prices quickly fell back below $2.

“Driving demand remains high, major refineries are still experiencing problems and we expect heavy refinery maintenance this autumn,” said Green.

“This means that it may take some time for the national average to reach $2 per gallon,” he said. “Nevertheless, gas-price declines should accelerate quickly by the end of September when people drive less and as stations switch over to less costly winter-blend gasoline.”

And if oil prices continue to drop, that “also could speed up the decline in gas prices,” Green said.

Some parts of the country could quickly see $2 gasoline at the pump, said James Williams, energy economist at WTRG Economics. That could happen in the “next few days depending on where you live,” he said. “Some refinery problems may delay it in the Midwest and Delaware.”

But he also emphasized that “California is not one of those parts” that will see $2 gas.

According to AAA’s Green, 12 states already had at least one station selling gas for less than $2 as of Monday. They included Alabama, New Jersey and Texas, and “the number of states with stations [selling gas for] under $2 is likely to grow fast in the coming weeks.”

Currently, South Carolina has the lowest average at $2.092 Wednesday, but drivers in California, which has been hit hard by refinery woes, are paying $3.429 a gallon, on average, according to AAA.