It's the most powerful A6, and it's designed to carry five adults and a great many of their possessions very quickly indeed while offering a modicum of economy.
The A6 Avant BiTDI's 3.0-litre diesel engine has two turbos and generates 309bhp and 479lb ft of torque, matching the 535d Touring's power output and beating its torque by 14lb ft. All of this oomph is transferred to the road through an eight-speed automatic gearbox.
What's the 2012 Audi A6 Avant BiTDI like to drive?
There are no external visual clues that you're driving a car that'll give hot hatches a real fright in a straight line.
There are few clues on the exterior to show what lies underneath the A6 Avant BiTDI's bonnet
The engine is slightly noisier than its less highly tuned stablemates, although part of this is undoubtedly down to the sound actuator that's designed to make things sounds sporty. Pull away and you're aware of a bit more vibration through the pedals than you normally get with Audi's V6 diesel engines, too.
The gearbox shuffles smoothly and unobtrusively up into the highest ratio it can, as soon as it can. Then you approach a roundabout, see a gap, put down your foot and not much happens. The A6 BiTDI takes a second to think about things, then the smaller of the two turbos wakes up and you're off at a fair lick.
The second turbo joins the fray at higher revs and makes the A6 Avant BiTDI genuinely rapid. The 0-62mph dash is over in just 5.3 seconds and the top speed is limited to 155mph.
All this pace makes the A6 Avant BiTDI a real overtaking machine. No longer are you held up by dawdlers: just squeeze the accelerator and lines of traffic are dealt with in seconds.
As standard, the A6 Avant BiTDI comes with Audi's Drive Select system, which allows the driver to choose from Comfort, Dynamic, Sport, Automatic or Individual settings. Unfortunately, in anything but Comfort, the ride is too firm, thumping over bumps that the 535d Touring has no issue with.
So you put the car in Comfort to soften the ride, but the body bobs and floats over even the smallest undulations. We ended up leaving the car in Auto and putting up with the ride.
What's the 2012 Audi A6 Avant BiTDI like inside?
The interior of the A6 Avant is a wonderful thing. Everything is made from the best of materials and has been put together with greater care than average.
There's loads of space for all five passengers, and the boot is ever so slightly bigger than that in the 5 Series Touring, whether the rear seats are up or down. The boot floor is perfectly flat, even with the rear seats down, and there's no lip to lift luggage over.
Standard kit is generous, and includes leather trim, cruise and climate controls, and electric windows all round.
Should I buy one?
If you're after the ultimate A6 Avant, we'd still recommend that you go for the single-turbo 3.0 TDI version, which is smoother, more fuel-efficient and not that much slower.
It also emits 156g/km of CO2, whereas the BiTDI model emits 169, so the lesser car sits two company car tax bands lower.
However, wed avoid both of these cars and go for the BMW 535d Touring. Its almost as quick, handles better than the Audi, rides better and has virtually the same space..
What Car? says