Diesel's street margin is 1.384 dollars a gallon, near its widest in a month

Diesel is the number to watch today. The retail-wholesale spread is 1.384 a gallon, which means the gap between what you pay at the rack and what you charge at the pump is about as wide as it has been all month. If you run diesel through a c-store or a hauling operation, that is real money on every gallon, and it is on your side right now.

The reason the spread has room is that wholesale diesel itself is 5.652, up 6.4 percent over 30 days and sitting right at the top of its range. When your cost is at a 30-day high and the street price has moved up faster, the margin widens. That only holds while retail keeps pace, so it is worth checking your own pump-to-rack gap against that 1.384 figure to see where you actually stand.

Gasoline is the other side of the picture. RBOB is 3.05, down 10.2 percent over 30 days and near the bottom of its range at the 29 percent mark. Cheaper gasoline feedstock plus expensive diesel is why the 3:2:1 crack spread tightened to 61.49, down 10.37 over the month. Refiners are earning less per barrel of combined product than they were, even with diesel strong, because gasoline is dragging the blend down.

Natural gas is quiet and cheap. Henry Hub is 2.888, and storage is full at 3184, near the top of its range going into the shoulder season. Nothing there is pushing your energy costs around this week.

The diesel spread is the thing to keep an eye on. Wholesale diesel at the top of its range could pull retail up with it, and if it does the current margin may narrow as your cost catches the street. Watch whether your rack price keeps climbing before you bank on today's gap holding.