2015: One great year for drivers at the pump

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American drivers, start your engines.

With crude oil prices falling and supplies continuing to rise, U.S. motorists are paying on average some of the lowest prices to fill up that they have in years.

The average price for a gallon of regular gasoline this year was the second-lowest of the last 10 years, according to AAA, which tracks national and local pump prices. Gas is cheaper than it's been since 2009, when the Great Recession sidelined millions of workers and prompted many car owners to cut back on driving.

The annual average price was $2.40 per gallon — or about 94 cents less than 2014. That saved Americans more than $115 billion this year, AAA estimates, or an average of more than $550 per licensed driver.

Oil market watchers don't see crude prices rising anytime soon — at least as long as the surplus continues to rise. Storage tank levels at a major hub in Cushing, Oklahoma, rose for the eighth straight week, hitting a record 62.9 million barrels. That news sent crude futures prices falling 3 percent.

Oil prices have fallen from a 2014 high of more than $100 a barrel to below $40 a barrel, trading briefly below $35 a barrel earlier this month.

Read More5 stocks to watch amid crude oil slide

Oil producers may soon run out of places to store the next barrel. With only 10 million barrels of working storage left at the Cushing hub, some oil traders have warned that space may soon run low. That could send crude prices even lower.

The crude oil glut is not evenly distributed around the country. While inventories are well above the five-year average in most regions, supplies on the west coast remain relatively tight.

That's one reason pump prices continue to vary widely across the country. (State taxes and transportation costs are also major factors.)

Some 71 percent of U.S. gas stations are selling for less than $2 per gallon, reported AAA, which noted that drivers in 41 states can find at least one station selling gas for less than $2. More than 16,000 stations across the country are selling gas for less than $1.75 per gallon.

But west-coast drivers are paying more. The five states with the highest prices per gallon at year-end were California ($2.85), Hawaii ($2.69), Nevada ($2.51), Washington ($2.47) and Alaska ($2.47).

California had the most expensive annual average of any state in 2015 for the first time on record. South Carolina had the cheapest annual average of any state for the fourth year in a row.