How do you build the world's most fuel-efficient production car? Start with the world's most aerodynamic design. The Volkswagen XL1 is capable of 261 miles per gallon thanks to its sleek shape and ultra efficient plug-in diesel hybrid powertrain. The downside is that only 250 will be made and none will be sold stateside. That didn't stop us from taking our turn behind the wheel of this truly revolutionary ride.