The average price for gasoline in the United States has fallen 4 cents a gallon over the past two weeks and 20 cents since the same time last year, a survey showed Monday. The Lundberg Survey of gas prices reported that a gallon of regular gasoline now costs $3.56. Midgrade costs an average of $3.74 and premium is $3.88. Of the cities surveyed in the Lower 48 states, Charleston, South Carolina, has the nation’s lowest average price for regular at $3.22. The Long Island, New York, region has the highest at $3.82, the survey said. In California, the lowest average is Sacramento’s $3.63. The highest is San Diego’s $3.82. Rounded off, San Diego’s price is the same as Long Island’s, but analyst Trilby Lundberg said that the California cities is actually a few percentage points of a penny less.