from wikipedia. I new that diesel is cheaper to refine but there are reasons for it sometimes having a higher cost than gas.

Fuel value and price

The density of petroleum diesel is about 0.85 kg/l (7.09 lbs/gallon) whereas petrol (gasoline) has a density of about 0.72 kg/l (6.01 lbs/gallon), about 15% less. When burnt, diesel typically releases about 38.6 MJ/l (138,700 BTU per US gallon), whereas gasoline releases 34.9 MJ/l (125,000 BTU per US gallon), 10% lear[4] by energy density, but 45.41 MJ/kg and 48.47 MJ/kg, 6.7% more by specific energy. Diesel is generally simpler to refine from petroleum than gasoline. The price of diesel traditionally rises during colder months as demand for heating oil rises, which is refined in much the same way. Due to its higher level of pollutants, diesel must undergo additional filtration[citation needed] which contributes to a sometimes higher cost. In many parts of the United States and throughout the UK and Australia[5] diesel may be higher priced than petrol.[6] Reasons for higher priced diesel include the shutdown of some refineries in the Gulf of Mexico, diversion of mass refining capacity to gasoline production, and a recent transfer to ultra-low sulfur diesel (ULSD), which causes infrastructural complications.[7]