I don't think there is anything other than a big diesel currently available that fits the bill. Maybe the best approach is to just run the A8 into the ground say another 5 years and hope that by then the development of electric cars will provide a suitable alternative.
Trouble is the 335d is great fun to drive, super performance and whilst it fits the bill perfectly today, may be worthless in 2-3 years time.
Decisions decisions!!!!!
Worthers +8
