Here in interior NorCal general purpose gas stations will price Diesel near premium 91 octane, at about $2,70 a gallon, Costco is the one retailer that keeps Diesel near 87 prices or ~$2,30 a gallon currently. So there is some nice profit taking on selling diesel. Definitely less competitive than the gasoline market in my part of the world.
I think this is the best answer I could find from our government's EIA division:Reference:Why are diesel fuel prices higher than gasoline prices?
On-highway diesel fuel prices have been higher than regular grade gasoline prices almost continuously since September 2004. This trend is a break from the previous historical pattern of diesel fuel prices usually being lower than gasoline prices except in cold winters when demand for heating oil pushed diesel fuel prices higher. There are three main reasons why diesel fuel prices have been higher than regular gasoline prices in recent years:
Demand for diesel fuel and other distillate fuel oils have been relatively high, especially in Europe, China, India, and the United States.
The transition to less polluting, lower-sulfur diesel fuels in the United States affected diesel fuel production and distribution costs.
The federal excise tax for on-highway diesel fuel of 24.4 cents/gallon is 6 cents per gallon higher than the gasoline tax.
Here is a retail price trend graph that further exhibits, the world demand picture on Low sulfur diesel:
Dreaming of Winter 2009, and 2016 .