In this week’s report on gasoline and oil prices from AAA, the average retail gasoline price dropped again from the week before, continuing a trend of falling prices since a peak of $3.94 per gallon (retail) on April 6. The average retail gasoline price nationwide was $3.69 per gallon as of May 21, 2012.

Although the national average gas price is falling, prices in some parts of the U.S., especially the West Coast, remain stubbornly high, even rising in some cases. For example, the average price on the coast, including California, was $4.24 per gallon, according to the U.S. Energy Information Administration (EIA). The average retail gasoline price, minus California, was still at $4.08 per gallon as of May 21.

During the last month, gas prices in California went up 14.1 cents, 12.6 cents in Washington and 15.8 cents in Oregon, despite falling prices east of the Rocky Mountains. Low gasoline supplies and refinery shutdowns and maintenance are driving this regional difference, according to AAA. The lowest average retail gas price on a regional basis was in the Gulf Coast, at $3.48, according to EIA. Overall, though, the average nationwide retail gasoline price is 20 cents cheaper than at the same time last year.

Diesel prices have remained more stable, down roughly 4 cents when compared with the same period in 2011. The average retail price of on-highway diesel has fallen from $4.05 to $3.95. California still has the highest on-highway diesel price at $4.30, but the lowest average retail diesel price, the same in the Gulf Coast states and in the Midwest, was at $3.96 (the average price on the West Coast without California was at $4.15) as of May 21.

Oil prices closed at $92.57 per barrel (for West Texas Intermediate, i.e. WTI) on May 21, now at the lowest price since Oct. 26, 2011. In addition, economic concerns in Europe (due to Greek sovereign debt concerns) and a slowing Chinese economy are driving down prices. OPEC also stated that it plans to increase oil production to bring the price of Brent crude to $100 per barrel, which would mean a corresponding WTI oil price in the mid-80s per barrel.