The average price of a gallon of gasoline in the United States fell by 9 cents in the past two weeks, driven by the latest round of falling oil prices, according to the Lundberg survey released on Sunday.

Prices fell to an average of $3.37 per gallon for regular grade gasoline, according to the survey conducted on Sept. 19. That extends a decline in prices to 13 weeks, survey publisher Trilby Lundberg said. The price of a gallon has dropped 34 cents over that period.

There has been a 13 percent slide in crude oil prices since June. While this decline has been driving the price of gas lower, two new factors caused a cut in prices at the pump over the past two weeks, according to survey publisher Trilby Lundberg.

Lundberg said that refiners have now shifted to winter blend gasoline, which is cheaper for them to make. Meanwhile, ethanol, which refiners blend into gasoline, has also seen price declines, which has factored into lower prices at gas stations.

These factors, along with more drops in oil prices, could keep gas prices lower in the short-term.

"Probably we will see a few more pennies drop at the pump," Lundberg said.