The spread between gasoline and diesel has been hard / impossible to explain for several years now. They were once pretty close together. As a member of the oil and gas community I am embarrassed by it since it does not have any real rational explanation. Diesel does have a higher BTU content but that does not explain the difference. With the high supply of near condensate shale oil diesel should be plentiful and relatively cheap. It is very easy to refine. Even the low sulfur does not explain the price difference and the additional tax on diesel does not either.
I see as much as a dollar spread.
I also saw diesel prices range from 3.25 to 2.45 on US 69/75 from Houston to W. Arkansas.
The switch from diesel to ULSD has been a reasonably steady difference in price to gasoline, until the last six months.
The refining into diesel is no longer simple with the EPA ULSD requirements. Even the sweetest crude needs a hydrotreater to make the specs.