Make no mistake: the price of the raw gas is about the same as the U.S., but Europe taxes gasoline at a higher rate. At the moment, taxes in France make up about 56 percent of the pump price. For comparison, the U.S. federal gasoline tax of of 2005 was 18.4 cents per gallon, with each State adding between 10 and 33 cents of tax, according to Widipedia. That makes the maximum gasoline tax rate 17% in the U.S. In actuality, the actual price of gas excluding taxes is pretty constant across countries: gas prices excluding taxes (read the first row).
How do I determine the current price of gas in Europe?
The U.S. Energy Information Administration tracks International Energy Prices weekly via graph of the data, showing data from 6 European countries. You'll find the following resources:
Note that Spain, Portugal, and Greece are not tracked in these graphs. All have lower prices than the tracked countries at time of writing.
Note that in many countries of Europe, Diesel fuel has less of a price premium to U.S. diesel. Some countries subsidize diesel through lower taxes. As a result, there are many more diesel vehicles in Europe compared to the US, and diesel fuel is more readily available.
If you're interested what the Ratio of Gasoline Price to World Average might be, nationmaster keeps track with Map & Graph: Energy: Gasoline prices.
How can I offset the high price of gas on my vacation?
Diesel fuel is widely available in Europe, is often cheaper than gasoline, and diesel cars get great milage. You might try renting or leasing a diesel on your vacation to save fuel cost. A small turbo diesel will accelerate like a champ and can get more than 50 miles to a gallon of fuel. Modern European diesel cars aren't your grandfathers smelly, black-cloud belching diesels; you probably won't know you're not driving a gasoline car.
You can save considerable money on gas by anticipating fueling stops and searching for the best prices. Remember, gas is sold by the liter in Europe, a much smaller unit of measure than the gallon, so a few Euro cents adds up fast in a fill up. You can often find cheaper gas in the huge hypermarkets that have sprouted up on the periphery of European cities.