New Audi A8 4.2-litre V8 TDI

Somewhere in the world, today probably, some clown is filling up the stonking Audi A8 4.2-litre V8 TDI's fuel tank with petrol.

That's because the filler has yet to recognise that this near silent, elegant, rapid and authoritatively-styled German is powered by a diesel engine. Getting out the petrol, flushing the fuel lines and replacing a few injectors and filters will be expensive.

Very expensive. But doubtless Audi service centres around the world are well practised. How can one blame the guy who filled it up? The A8 4.2TDI idles with almost inaudible assurance, responds instantly to a touch of the accelerator and cuts through freeway traffic with so little fuss and bereft of noise that the clink of Armagnac flutes from the rear seat can clearly be heard.

All this doesn't come for nothing. The limousine will cost $238,500 before it's ready for the road. It is loaded with equipment that will keep you amused for hours - possibly days. If you let children loose in here, you may not see them for months.

All this before you turn over the engine. Standard kit starts with the best materials and finest construction ever to grace the cabin of a sub-$500,000 car. It's brilliant.

There's also a superb audio and sat-nav system, a touchpad with handwriting recognition, wireless for your iPad/iPhone or Android and Audi's Multi Media Interface that does everything from alter the drivetrain characteristics to help lower fuel consumption.

It's a lavish and expansive saloon with its hard bits built predominantly from aluminium - it weighs about 40 per cent less than a comparable steel body - and the soft bits from leather, padded vinyl and wool-blend carpet. It has family lines but while its possible to mistake it for an A6 on a damp night, it is clearly too much a limousine to be an A4. There's more room than most people deserve and even the boot is big enough to carry sufficient cash to buy a small country.

The A8 rides on adaptive air suspension and the bi-turbo V8 diesel drives all wheels through an eight-speed sequential automatic. Remarkably, the 4.2-litre engine pumps 258kW/800Nm - that's not a mistake - and yet can get 7.6 L/100km.

Not when you max it out over the 0-100km/h spring, however, which can be dashed in a mere 5.5 seconds. Not bad for a 2-tonne, 5.1m top-notch four-door sedan. The MMI also gets a nod for its technological excellence.

This should go without saying but the A8 is the pinnacle of Audi's safety program. Some of the equipment is optional - and no surprise here but it's expensive - but the standard lifesaving aids should be more than adequate. The A8 previews Audi's pre-sense safety system for avoiding accidents and minimising the consequences, and gets night vision assistant with pedestrian marking and adaptive cruise control with "stop and go" feature.

Forget the word diesel and immerse yourself in 800Nm of torque flowing seamlessly to all wheels through a silky ZF eight-cog auto. Wet corners at night dissolve as a dry corner in day with the limpet grip of the tyres and AWD and the canny electronic aids. The quietness is so unnatural that the experience of cruising country highways is almost surreal.

Back to reality quickly, though, when overtaking and feeling that torque thrust the car forward without a murmur. Practical car for the city? Probably not. Perfect open-road tourer - also a maybe given it has no proper spare wheel. But the liquidity of its forward motion and readiness to spring to life without flinching, is magic.

More Pictures of New Audi A8