WASHINGTON -- U.S. retail gasoline prices fell 15 cents a gallon over the last week, dropping below an average $3.50 a gallon for the first time in six months, the government said Monday.

The national price for regular unleaded gasoline is $3.48 a gallon, up 71 cents from a year ago but the cheapest since April 14, the federal Energy Information Administration said in its weekly survey of service stations.

Pump prices are falling due to a combination of cheaper crude oil and declining gasoline demand.

U.S. crude oil futures settled Monday at the lowest level in eight months at the New York Mercantile Exchange, dropping $6.07 to $87.81 a barrel.

Crude has declined some $60 a barrel, or $1.42 a gallon, since hitting a record high of $147 a barrel in July. However, gasoline has fallen only 63 cents a gallon since then, so there is room for more of those oil savings to be passed on to consumers at the pump.

At the same time a weak economy is cutting into industrial use of petroleum products and consumer gasoline demand is down 4.5 percent in the last month compared to a year ago.

In the EIA's weekly survey, gasoline was the most expensive in the lower Atlantic states at $3.67 a gallon, down 10.9 cents. Chicago had the highest city price at $3.75, down 16.2 cents.

The New England states had the lowest regional price at $3.39 a gallon, down 14.4 cents. Cleveland had the lowest city pump price, down 33.5 cents at $3.28.

The EIA also reported gasoline prices were down 12.7 cents at $3.68 in Miami, down 7.3 cents at $3.56 in Los Angeles, down 10.7 cents at $3.54 in Seattle, down 10.3 cents at $3.46 in Denver, down 13.5 cents at $3.40 in New York City, down 16.6 cents at $3.35 in Houston and down 14.3 cents at $3.32 in Boston.

Separately, the average price for diesel fuel declined 8.4 cents to $3.88 a gallon, the lowest since early March but still up 84 cents from a year earlier, the EIA said.

The New England states again had the most expensive diesel at $4.04 a gallon, down 4.1 cents. The Gulf Coast had the cheapest fuel at $3.84, down 8.6 cents.