Various bodies track the pump price of fuel across different European countries. Britain's AA publishes a monthly survey of reported costs, covering both the UK regions and other countries. Apart from showing a lot less variation between UK regions than you might expect, it highlights the predictable (Norway obscenely expensive, Spain and Poland pretty cheap), but also some surprises: the Dutch consistently have to pay the most for petrol, whereas just about the cheapest fuel in Europe is not that far away, in Luxembourg - see side panel on the Wonder that is Luxembourg.
Update 23 October 2017:

World oil prices are just above $50 a barrel right now, and petrol (not diesel) in much of the near Continent is starting to look pretty expensive to us poor Brits. From having long been one of the most expensive places in Europe to buy petrol, the UK is now close to the middle of the range: you'll pay more for petrol in eleven other EU countries, including all our main neighbours. Our pump price is now only 4% above the EU average. Obviously the weak pound is the main reason pushing up the impact of Euro-denominated prices. Is it a tribute to our competitive supply chain here that it's taking quite some time for prices to climb back up here? After all, it's the same product everywhere, sourced on the same world markets, and we do still slap a lot of tax on top here.

As for
diesel, the UK has lost its place as having the highest pump price in Europe; it still costs 14% above the EU average, but Italy, and now Denmark, Sweden and Belgium all come in a little higher still. Diesel prices here are back to being a little above our petrol price - whereas just about everywhere else (for policy reasons best left forgotten) charges less tax on diesel than on petrol.

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High fuel prices in recent years have been cited as a reason not to take the car to the Continent. But with oil now priced at around $50 per barrel, car travel is competitive in cost terms with other forms of travel. And a bit of planning - using the right sort of petrol stations, and more importantly, where possible, filling up in the right countries, can lead to some worthwhile savings. Price comparisons with the UK are very much steered by the exchange rate, but we're usually amongst the more expensive countries.

The picture is particularly bad for diesel: most countries levy a lower rate of tax on diesel than on petrol, in part because of the strength of the freight lobby. The UK Government makes no such distinction, recognising that diesel is in fact still more damaging to the local air quality than petrol - as the VW scandal has highlighted. (The traditionally higher pump price of diesel in the UK simply reflects the higher production costs and lesser economies of scale.)
Differences in tax policies are of course the main reason for the wide variations in fuel prices. The consultancy firm European Energy Portal runs a fascinating and helpful website - excerpt above - comparing prices per litre across the Community, which goes on to show how far it is Governments and how far it is the oil companies themselves that are raking it in when you pay at the pump (see the more detailed table on fuel taxes further down the website).

Advice on where to buy
within a country is pretty much the same as you would follow at home: try to avoid the two extremes of service areas on motorways (whether you've had to pay for the privilege of using the road or not) and isolated rural facilities. Better to seek out the more suburban sites which the majority of locals go for, including of course the supermarkets. These are particularly advantageous in France: the hypermarkets outlets may be shabby, tricky to access within the development, and often inefficient (many still run a cumbersome security system which only unlocks the pump for your use when the previous customer has got to the head of the queue and paid), but the saving of a euro or two on a full tank is probably worth the effort. Some countries, such as Luxembourg and Croatia, have fixed prices (how does this get past the competition authorities?).

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On any route that takes you through eastern France, southern Belgium or into central Germany it's worth assessing whether it's worth making a modest diversion to pick up the delights of Luxembourg. The northern half of the Grand Duchy has pleasant wooded scenery and small towns clustered around ruined castles; there are attractive drives through vineyards along the Mosel in the east (and the chance to stop off in the village of Schengen, hated by barrier manufacturers across Europe); but the rest of the country is rather duller, and still heavily industrial along the French border, although the capital offer some compensation if you like gorges.

But the main reason to stop off in Luxembourg is to fill up with petrol, for some 20 cents a litre cheaper than in neighbouring Germany. It's a very high cost country otherwise, so the low fuel price is purely a result of tax differentials. Allegedly it's the consequence of government policy to support the Grand Duchy's own potentially disadvantaged border regions (yes, even Luxembourg needs a regional policy).

By the way, the price of fuel is fixed by the Government, and so, unusually, there is nothing to be gained from seeking out petrol stations off the motorway. Best go with the flow and fill up on the (free) motorways. The Aire de Berchem service station just south of Luxembourg City (pictured) has reportedly the highest fuel sales of any retail outlet in the world - over 200m litres per year). Or aim to head through the remarkable strip of petrol stations on the eastern, Luxembourg side of the road in the ugly village of Martelange on the main N4 through the Belgian Ardennes - photo here:
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