I don't know where you got those figures from.
Maybe we should ask Mythbusters to test it.
It's basic physics - higher speed has higher losses in friction and air and rolling resistance and they increase exponentially with speed. If you drive in top gear the slower you drive the better efficiency is down to the lowest rpm that engine is happy to with which would be around 1200rpm on A8 and 1500rpm on S8. The question is can you get better economy if you drive in lower gear as losses to drive engine and ancillaries remain the same even if air and rolling resistance drops. Please try and report back, I am sure it will be below 50mph and most likely 20-40mph range.
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Currently 8less
2011 Q7 S Line 3.0TDI, 2016 Tesla Model S 90D
8 history:
2006 A8 Sport 4.2TDI quattro SOLD,
1997 S8, reached end of life with gearbox failure
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