In some countries that share a border, abridge or a tunnel, the difference in the price of gas can be substantial. A driver in Britain, where fuel can cost $5.50 a gallon, can save as much as $1.80 a gallon by tanking up in France. Germans can pay $3.70 a gallon-or seek out gas in Poland for 75 cents less. Of course, not every European lives close to a border, so the economic impact of those shopping for cheaper gas is minor. But the crossborder odysseys play into a much bigger debate over the environment, the economy and, increasingly, the Continent's politics.