How much of what I pay is tax?
Almost half of what you pay at the pump is tax. What a litre costs splits into three: the fuel itself with the station's margin, excise duty —a fixed amount per litre, the same whether you put in 20 litres or 50— and VAT, charged on top of the other two. With petrol at 1.50 € a litre, around 70 cents go to those two taxes. Here you can see the exact split for each fuel and each date.
Why does petrol go up when oil goes down?
Because oil only moves one of the three parts of the price. Excise duty is a fixed amount per litre that takes no notice of the market, and VAT is worked out on the total. With petrol at 1.50 €, if the fuel itself gets 20 % cheaper, the drop at the pump comes to 13 %; and if taxes go up that month as well, you can end up paying more even though oil is cheaper. Here you can see which of the three parts actually moved.
Where do these prices come from, and what do they not tell you?
Prices come from the Spanish ministry's official list of stations, and the average is the plain one: the prices of every station that reports are added up and divided by the number of stations. That means a motorway station selling a huge amount counts the same as a village one selling very little, so it is the average price of a station and not what Spain pays on average. And the tax subtracted is the one in force on the date of each figure, not today's: for a price from 2013, the 2013 tax is used.