Audi offers a choice of two V6 engines: a 335bhp 3.0-litre that runs on petrol (called the 55 TFSI) and a 3.0-litre with 282bhp that runs on diesel (badged 50 TDI).
The diesel is smooth and quiet, if not quite as refined as the 400d engine in the rival Mercedes S-Class. It isn't as powerful as that engine, either, although performance is more than adequate and there's a healthy swell of acceleration from low revs.
For those worried about the future implications of buying a diesel, the petrol makes a wholly convincing alternative. True, the 55 TFSI doesn't have quite the low-rev grunt of the diesel, but it's still well suited to the A8. Its power delivery is buttery smooth and you barely hear any noise from it, especially when in Efficiency mode. Engage this and the engine turns off automatically when you lift off the accelerator at motorway speeds, allowing the car to enter a fuel-saving 'coasting' mode.
This feature is enabled by a relatively big 48-volt battery. It powers all the auxiliary functions, such as the air conditioning and power steering, and allows up to 40 seconds of coasting before the engine comes back online, entirely unnoticed.
When you want to get a shift on, the petrol engine revs more keenly and is ultimately faster than the diesel, although the equivalent petrol in the Mercedes S500 is quicker still. Whichever engine you choose, you get an eight-speed automatic gearbox, which can be slow-witted; it often causes a delay in the acceleration to arrive after you've pressed the accelerator pedal.
Audi’s quattro four-wheel drive system comes as standard on all versions, providing impressive all-weather traction and reassurance that offers a distinct advantage over rear-wheel-drive versions of the S-Class and BMW 7 Series.
There’s also the option of four-wheel steering. At low speeds, this steers the rear wheels in the opposite direction to the fronts, improving manoeuvrability and making the A8 feel shorter than it is in tight spots, such as multi-storey car parks. At higher speeds, the wheels move in the same direction for greater stability when, for example, changing lanes on the motorway. As a result, the A8 is as able to fill you with confidence at high speeds as it is to keep you relaxed around town.
While it is easy to drive and very surefooted, the A8's big body wallows on its standard air suspension through bends; it definitely prefers going in a straight line. Put simply, the rival S-Class is much more enjoyable to drive – especially on a road with lots twists and turns along it.
But, of course, pointy handling isn't really what the big Audi is all about, and the A8's softer edge has an important upside: it's not only the best-riding car in its class, but one of the most comfortable cars full stop. It simply glides along any stretch of road, seemingly untroubled by all but the most pronounced bumps. Whether you’re sitting in the front or the back, you’ll enjoy unparalleled levels of comfort.
Granted, on 20in wheels the A8 lets in a hint of road noise over coarse surfaces, but wind noise is kept to a minimum at motorway speeds. If you want even greater separation from the outside world, you can option double-glazed side windows for a reasonable cost.