Diesel power ehh...
 

Top gear clips here with A8 4.2 diesel doing 800m to the tank:
part1

<iframe width="560" height="349" src="http://www.youtube.com/embed/y9faxMTaOCM" frameborder="0" allowfullscreen></iframe>

and part 2

<iframe width="560" height="349" src="http://www.youtube.com/embed/E5SCfsALPug" frameborder="0" allowfullscreen></iframe>

After replacing wife's Freelander with a 2001 A6 2.5TDI Quattro Sport Avant I hardly get in the S8, I just drive her car. Comparing A6 to S8 it is 33mpg vs 18mpg. And that's what on-board computer tells in real life it is more like 38 vs 18. And due to the way I fill up A6 it works out 15 miles/£ on A6 vs 3 miles/£ on S8