Diesel could hit $10 a gallon by December, which would be bad news for truckers, farmers, and anyone who buys goods shipped by truck — meaning nearly everyone.

• The warning rests on the "crack spread," the profit a refiner makes turning a barrel of crude oil into gasoline and diesel.
• That spread is unusually high right now, which means buyers are desperate for diesel and willing to pay a big premium for it.
• Wall Street analysts like JPMorgan are not making the same call, but the market's own pricing points that way.
• A diesel spike would push up shipping and farming costs, feeding into higher prices for food and goods.

Outlook: If refining margins stay this high into winter, diesel prices are set to climb sharply, adding fresh pressure to inflation.