Diesel's retail margin fell about 37 cents a gallon over the past month, to $1.104

The spread between what diesel sells for at the pump and what it costs wholesale is 1.104 now, down 0.367 in 30 days. That is roughly 37 cents a gallon gone from the retail margin in a month. If you sell diesel, you are working on a thinner cut than you were in early August, even though the street price is high.

The reason is in the two diesel numbers. ULSD diesel futures are up 17.1% to 4.639, sitting at the 97 percent mark of their 30-day range. Retail diesel is up only 4.7% to 5.599. Wholesale ran up more than three times as fast as the pump, so the gap that pays your overhead got squeezed. Both prices are near the top of their range, retail at the 98 percent mark, but the wholesale side is the one that moved, and that is where the margin went.

Crude backs the same story. WTI is 93.89, up 19.1% in 30 days, and Brent is 98.69, up 16.8%. When the barrel climbs this fast, refiners and wholesalers reprice ahead of the street, and the retail margin thins until pump prices catch up. The 3:2:1 crack spread is 56.79, down 4.1 over the month, so refiners are feeling a smaller version of the same pinch on the making side.

Natural gas is the quiet one, 3.009 and up 10.5%, right in the middle of its range at the 52 percent mark. Nothing there to price around today.

Retail diesel is already at 98 percent of its 30-day high, so pump prices have little room left to run before demand pushes back. If wholesale holds near 4.639, the retail margin could stay tight until the street price catches the barrel, which may take a few weeks.