Two fuels, two different markets
Petrol and diesel leave the same barrel of crude, but they are sold into different worlds. Petrol demand is dominated by private cars. Diesel competes with the freight industry, agriculture, construction and — in cold months — with heating oil, which is chemically a close cousin. When trucking activity is strong or a cold winter raises heating demand, diesel firms up even while petrol stays flat.
Why diesel often costs more in the United States
The U.S. Energy Information Administration, which publishes weekly retail prices for both fuels, notes that U.S. on-highway diesel has generally priced above regular gasoline for the past two decades. The reasons it lists: sustained demand for diesel and other distillate fuels, the extra refining cost of ultra-low-sulfur diesel, and a federal excise tax that is 6 cents per gallon higher on diesel than on gasoline.
Why diesel is often cheaper in parts of Europe
In much of Europe the picture historically flipped: many governments chose lower excise duty for diesel than for petrol, which can more than offset diesel's market premium at the refinery gate. Whether diesel ends up cheaper at the pump in a given country is therefore mostly a tax decision, visible country by country in the European Commission's Weekly Oil Bulletin, which lists both fuels with and without taxes.
The seasonal wave
Diesel has a winter season. Because it shares its refinery stream with heating oil, cold snaps in Europe and North America pull the same molecules in two directions, and pump prices follow. Petrol's seasonal peak, by contrast, tends to arrive with summer driving demand. If you drive a diesel and can choose when to fill a large tank, the calendar is quietly on your side in spring and early autumn.
Check, don't assume
The honest answer to "which is cheaper?" is: it depends on the country and the week. Refuelia's fuel price pages link the official source that covers each market, and station-level prices appear only when a verified feed or a driver report exists — with a timestamp, never an estimate.