The pump price is four prices stacked together
Every price you see on a European forecourt is built from the same four layers: the cost of the refined product itself (which tracks international markets and is broadly similar across the continent), the distributor's and station's margin, national excise duty, and VAT applied on top of everything — tax on tax.
The first two layers don't vary that much between neighbouring countries. The last two vary a lot, and that is almost the whole story of why crossing a border changes the price.
Excise duty: the EU sets only the floor
EU law — the Energy Taxation Directive (2003/96/EC) — sets minimum excise rates: €0.359 per litre for unleaded petrol and €0.330 per litre for diesel. Every member state is free to tax above that floor, and most do, some by a wide margin. That single decision, taken country by country, creates most of the gap you see at the border.
VAT multiplies the difference
VAT is charged on the fuel plus the excise duty, and standard VAT rates across the EU range roughly from 17% to 27% depending on the country. A higher excise duty therefore gets amplified by a higher VAT rate — which is why the spread between the cheapest and most expensive EU countries is bigger than the tax tables alone suggest.
Where the official numbers live
The European Commission publishes the Weekly Oil Bulletin — consumer prices of petroleum products, with and without taxes, for every member state, updated weekly. It is the reference dataset for comparing countries honestly, and one of the sources Refuelia links to on country pages where official data exists.
What this means for your tank
If your route crosses a border, it is worth checking both countries' current levels before you decide where to fill up. Refuelia shows stations on both sides of any border on the live map, and country pages show which official price source covers each market. Where we have no recent data for a station, we say so — we never estimate a price.