In different countries of Europe prices of fuel often differs much. Thus, while crossing Europe by car, it can be smart to optimise where you refuel. You can save much by fueling just before leaving cheap country, and reduce fueling in exepensie ones.
Additionally you may want to get off motorway (where fuel is always most expensive), and use smaller gas station. Often cheapest are these fully automatical (no staff at all), near shopping centers, etc...
Fuel prices in € per liter: