I watched Harry's Garage video on new Defender with the D300 powertrain and his comparison to the Range Rover Sport with the same powertrain and his explanation of why the Defender used 1/3 more fuel. Higher weight was (a small) part, the drag coefficient the main one. This was in relation to motorway cruising speeds. The difference between your A8 and Macan mpg could be similarly accounted for in the same scenario? That said, off the motorway I'd expect the regen braking of the A8 to contribute to the 20% fuel saving. You,, knowing the driving patterns, would know better as to which of the two (superior aero, regen braking) is making the difference. Possibly/probably both - benefits whatever the driving conditions. If so, that IMO justifies any additional complication.