This is the official answer to this question from the US Department of Energy:
Historically, the average price of diesel fuel used in motor vehicles has been lower than the price of regular gasoline. However, this is not always the case. In some winters where the demand for distillate heating oil is high, the price of diesel fuel has risen above the gasoline price. Since September 2004, the price of diesel fuel has been generally higher than the price of regular gasoline all year round for several reasons:
* High worldwide demand for diesel fuel and other distillate fuel oils, especially in Europe, China and the U.S. and a tight global refining capacity available to meet demand.
* The transition to low-sulfur diesel fuel in the U.S., which is affecting diesel fuel production and distribution costs.
* The Federal excise tax on diesel fuel is 6 cents per gallon higher (at 24.4 cents/gallon) than the tax on gasoline.
