Five of the most efficient cars in their class

This week, Earth Day attempts to inspire the world's inhabitants to think greener. With the price of fuel rocketing and an increased awareness of our effect on the environment, car buyers are focusing on models that use less fuel and produce lower emissions. Consequently, the car makers are clamouring to produce the most efficient cars in their class. Here we pick five popular market segments and the car in each that is the most efficient when compared against its contemporaries. That doesn't always mean going just by the numbers though.