Because breaking a barrel of crude into its component parts yields ~44% gasoline and only 29% distillates (Diesel), refiners run a lot more gasoline than they need to supply the diesel the market demands.  Inventories of diesel are below their 5-year average by 11%!   Hence producing one barrel of gasoline yields about $1.50 as of the Friday before Christmas, compared with an average of $12 going back to 2012.  In contrast, diesel margins were recently at $19 a barrel, compared with a longer-term average of $15.