The sixth-generation Audi A6 sedan has been unveiled, trailing its Avant wagon sibling by a month-and-a-half. With a co-efficient of drag of just 0.23, the new A6 sedan is said to be the most aerodynamically efficient petrol- or diesel-powered Audi to date. These new sedan and wagon A6 models are powered by turbocharged petrol or diesel engines, and shouldn’t be confused with the A6 e-tron range, which is all electric and largely unrelated. Know the news with the 7NEWS app: Download today Audi initially planned to bestow the A7 on badge the new petrol/diesel A6 range, but has since walked…