I have a friend traveling a couple of weeks in France and Spain. He was asking me about the better places to fill up the tank and I sent him this answer:
The problem is that he is not so much worried with price difference between countries but more about the price inside the country, since it may vary from station to station up to 12 cents/liter or even more. Considering he uses Diesel, this means over 10% difference.
Are there resources that compare petrol station prices in France and Spain?