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I remember when diesel was always cheaper than gasoline.
Which cost more to refine?
And also from a refiners point of view, the feed stocks (Crude Slate) must be matched to optimize either for gasoline or diesel. Certain feeds produce more of either product. Probably the refineries have balanced the feeds in anticipation for "Driving season", in other words cars on vacation and such. Now that didn't happen so an abundance of gasoline is in the system. This process is done over a period of months, it just doesn't happen in a day.
Diesel demand remains fairly steady year 'round.
And don't forget about ULSD, which requires the addition of lubricity additive to compensate for the loss of sulfur. Not cheap to do, and is done at the point of distribution (terminal level).
So ultimately, diesel fuel is more costly to make and distribute.