The price of gasoline ahead of the Labor Day weekend has reached the lowest level in 11 years in federal data and fallen below diesel prices for the first time in more than four years in WEX Inc.'s fuel index.

The national average retail price for gasoline reached $2.51 per gallon on Aug. 31, which was the lowest average price before Labor Day since 2004 and 95 cents lower than a year ago, according to data from the U.S. Energy Information Administration.

The U.S. average retail gasoline price increased 47 cents per gallon since the last week of January, when it was $2.04 per gallon, driven by rising crude oil prices earlier in the year, strong demand during the summer driving season, and unplanned refinery outages on the West Coast and more recently in the Midwest.

At the same time, the average price of diesel fuel fell below the price of gasoline for the first time since July of 2011, according to WEX. The average price of $2.59 for gasoline was above the average price of $2.58 for diesel on Aug. 15, WEX data shows.