Why is the Audi A2 considered to be a revolutionary car in terms of fuel efficiency and sustainability?

I've heard a lot about the Audi A2 being the first premium car to have an extremely low average fuel consumption of just 3.9 liters per 100 kilometers and its use of innovative materials for its lightweight construction. How did Audi achieve this and what makes the A2 stand out in terms of sustainability compared to other cars in its class?

One of the main reasons for the A2's exceptional fuel efficiency is its aerodynamic design, which was achieved through countless wind tunnel tests. This, combined with the use of lightweight aluminum in its construction, makes the A2 significantly more fuel-efficient than its rivals. As for sustainability, the A2 also incorporates recycled materials and uses eco-friendly production processes.