AAA says the spread between ULSD and RUG is about 27 cents nationally. And 5.2 cents in California.
The Energy Information Agency, part of the Dept of Energy, says the spread is 15.3 cents between ULSD and RUG. In California, the ULSD is actually cheaper than RUG by 4.5 cents, 214.4 cents versus 218.9 cents.
The EIA uses the volume of sales in its calculations, I think, so the cheap station that sells 5000 gallons a day of diesel does not count the same as the expensive station that only sells 200 gallons a day. AAA does not care how much is sold at each price, only what the price is on the pump.
In early December, I predicted that ULSD would be cheaper than RUG, on a national average, by May 1st. Now I am almost certain it will happen sooner, possibly before April 1st.
I have also predicted that ULSD would be at least 30 cents and possibly 50 cents a gallon cheaper than RUG by the Fourth of July. I should be a little worried about this prediction, as some might think it is impossible for diesel to get that much cheaper than gasoline. Yet that's what I predicted, now we'll just wait and see if it comes true.