On-highway diesel prices took a somewhat steep drop last week after falling only slightly the week before.

Nationally, prices came down 6.2 cents to average $3.888 per gallon. California dropped a full 9 cents, but remained the most expensive state at $4.146. Every region experienced price drops.

U.S. oil futures fell slightly in thin trading Monday, as concerns about Greece's debt crisis and U.S. demand kept a lid on prices. Brent crude, however, gained ground, sending the gap between the two major contracts rising for the first time in three sessions.

The upcoming vote on austerity measures in Greece has the market on edge, as a failure to pass the cuts would jeopardize bailouts and hence threaten global crude demand.

Last week's decision by the International Energy Agency also affected crude prices as additional oil from the Strategic Petroleum Reserve came on market. It will likely push prices down for the next several weeks.