For a diesel-powered limo weighing substantially over two tonnes as tested, the best-selling Audi A8 4.2 V8 TDI’s performance is frankly preposterous.

Making full use of its all-wheel drive system, that 590lb ft of torque and its army of tightly stacked gear ratios, it hits 60mph in 5.0sec flat and 100mph in 13.0sec, numbers you’d have needed a supercar to match 10 years ago. It is the fastest-accelerating diesel car we have tested, and by a huge margin.

The A8 is stunningly refined, especially at a motorway cruise

Yet the figures themselves aren’t half as impressive as the manner in which they’re delivered. A slight grumble at idle is the only way most occupants would ever twig which fuel feeds this engine. It’s whisper quiet at a constant cruise, with just a hint of a snarl when it gets down to business. Truth is, if you never did more than 2500rpm, the A8 would still provide all the performance you’d ever really need.

We love the way it interacts with the ZF gearbox, too. It’s quick to leap down a cog or two when you call for it, but the rest of the time it’s happy letting the engine’s torque do the work, keeping the revs low and the changes so smooth that they’re noticeable most from the repositioning of the revcounter needle.

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Our only concern is an occasional and slight awkwardness around town if you almost come to a halt but then have to set off again.

The 3.0-litre V6 diesel is similarly refined and packs quite a punch combined with impressive smoothness. Plus, with eco measures such as stop-start, it’s reasonably fuel efficient. For most drivers (or owners) most of the time it delivers more than sufficient performance.

The diesels more or less match the acceleration figures of their similar-sized petrol brethren, but it’s the extra torque (combined with economy) that makes the petrol cars redundant – especially the W12-powered car. We'd only recommend the petrol route if you cover very few miles and must have the absolute last word in refinement.

The W12 as you would expect packs a punch and is far more soulful than its diesel compatriats, while the snarling 4.0-litre V8 under the bonnet of the S8 turns this limo into a 2.0-tonne bullet.

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