regular unleaded. Over the years it crept up and it seems to me it was in the early 90's when parity began to be reached between regular and diesel. I had long thought much of it had to do with taxation, as an over-the-road truck will typically put on 200 or more gallons at a fill up and that represented and enormous revenue opportunity for the various states. In the old days, trucks used to have to have a fuel tax sticker for each state they operated in and purchased fuel in. Now that complex system has been replaced with the "IFTA" sticker or "International Fuel Tax Agreement (or Association)
Here is the page from their web site which lists the tax per gallon assessed by the various states and Canadian Provinces;
http://www.iftach.org/taxmatrix3/choose_tableq2.phpBut that list does not really answer the core question - why is diesel so high?
It has as much to do with the switch to low-sulfur diesel than anything, I think. But I confess, I am not qualified to answer your questions authoritatively.
I am not a petroleum engineer and I have never worked at a refinery (though I think there are several DU'rs that have) but I read an article recently which noted that lately we have been importing diesel from Europe. Yup, ocean going tanker ships coming to the states full, not of crude oil but of diesel, ready to be pumped into a truck. There are many reasons this is happening, from maintenance on US refinery's to seasonal formula switching to the old "NIMBY" (Not In My Back Yard) and "BANANA" (Build Absolutely Nothing Anywhere Near Anything) arguments against building new refineries.
It's a combination of factors; Increased worldwide demand (expansion in China and India means more goods moved by diesel powered transport), the switch to low-sulfur diesel in this country, generally tight refining capacity and taxes.