If you've ever pulled up to a pump and noticed diesel is 30 to 60 cents more per gallon than regular, you're not imagining things. Diesel costs more than gasoline most of the time, and it's been that way consistently since about 2004.
Before that, diesel was actually cheaper. So what changed?
Diesel demand went global
Diesel powers the world economy in a way gasoline doesn't. Trucks, trains, ships, construction equipment, farm machinery, generators. In the US, about 75% of all freight moves by truck, and those trucks run on diesel.
But the bigger shift happened overseas. Europe moved heavily toward diesel passenger cars in the 2000s. China and India industrialized rapidly, building infrastructure with diesel-powered equipment. Global demand for diesel grew faster than gasoline demand, and refineries had to adjust.
Refining crude oil produces a mix of products. You can't just make more diesel without also making gasoline, jet fuel, and other stuff. When diesel demand outpaces gasoline demand globally, the price of diesel gets bid up relative to gas.
Taxes add to the gap
Federal diesel tax is 24.4 cents per gallon. Federal gasoline tax is 18.4 cents. That's a 6-cent gap before state taxes even enter the picture. Most states also tax diesel at a higher rate than gasoline.
The reasoning is that heavy trucks cause more road wear than passenger cars, so diesel taxes are set higher to fund road maintenance. Fair or not, it adds up.
Winter makes it worse
Diesel and heating oil are essentially the same product. During cold winters, heating oil demand surges in the Northeast and Midwest. That pulls from the same refinery output as diesel fuel, tightening supply and pushing diesel prices up. You can see this clearly in the FuelWatch historical data if you compare regular and diesel prices during January and February of cold years.
When does diesel get cheaper than gas?
It's rare, but it happens. The last time was briefly during COVID in 2020 when trucking demand collapsed. If you see diesel drop below regular gasoline, it usually means the economy is slowing down hard. Freight volumes have fallen, construction has stalled, and refineries have more diesel than the market needs.
It's actually a useful economic indicator. The diesel-to-gasoline price ratio tells you something about industrial activity and freight demand that stock market indexes sometimes miss.
What this means for truckers and fleet operators
Diesel price swings hit harder when fuel is your second-biggest operating cost after labor. A 50-cent increase on diesel costs a long-haul trucker an extra $150 per fill-up on a 300-gallon tank. Multiply that across a fleet and it's serious money.
That's why price alerts exist on FuelWatch. Set a threshold for diesel in the states where your fleet operates, and you'll get an email when prices cross it. It won't make diesel cheaper, but at least you won't be surprised.
You can track diesel prices for any state on the FuelWatch map. Click a state to see both regular and diesel prices, with historical data going back to the 1990s.