The weekly retail on-highway diesel prices released by the Energy Department Monday (July 9) show the cost of diesel fuel continues to slide. The national average price per gallon dropped for the sixth straight week to $1.39 - the cheapest diesel has been in more than three months.

The biggest drop from a week ago was in the Rocky Mountain region. Diesel there decreased about 6 cents to about $1.44 per gallon. The cost of fuel in the region has plummeted more than 18 cents in the past five weeks.

The lowest prices nationally are in the Lower Atlantic region. Fuel there decreased about 2 cents from a week ago to $1.34 per gallon.

The cost of diesel in the East Coast, New England and Central Atlantic regions dropped to their lowest prices in more than one year. Diesel fuel in New England dipped to $1.50 per gallon - the cheapest it's been there since May 2000. Fuel is at its cheapest since January 2000 in both the East Coast ($1.39) and Central Atlantic ($1.47) regions, respectively.

The highest prices nationally are found in the state of California. Fuel there costs $1.61 per gallon.

The remaining regions' price per gallon is as follows: Midwest, $1.37; Gulf Coast, $1.36; and West Coast, $1.53, respectively.