A dark grey day. A dark grey road. In a dark grey car I'm approaching them quietly along the hotel drive. It's a diesel executive car. It doesn't sound like the opening stages of a thrilling drive… but this is the Audi A6 twin-turbo TDI quattro – described by the maker as the most potent production V6 diesel it's ever made. Is that the distant rumble of an approaching storm I hear?The seven double-spokes on the big 19" alloys are paddling sedately along the drive towards the main road. I'm settling into the black leather seats and casting an eye over the panel of controls around me. My mind is clicking through the fact sheet I've just read. I'm dwelling on the promise of something special.Rise up through the numbers. One saloon car, two turbochargers, three-litre capacity, four-wheel-drive, five-second 0-62mph sprint, six cylinders.Whirring under the bonnet is an engine capable of turning out 309bhp and a stump-pulling 480lb/ft of torque. That's all fed through a close-ratio, eight-speed (yes eight-speed) automatic Tiptronic gearbox to the front wheels most of the time, but to all four wheels when it's needed.Amazingly enough, we're also told this car is capable of an average fuel consumption of 44mpg which means, given its 16.5-gallon fuel tank, it has a potential range of over 700 miles. That's Penzance to Aberdeen.Penzance is not in my mind just now. I’m at the end of the drive. The indicator is ticking at me. The road is clear. I pull out, level the car at the hilltop ahead of me, and stand on the throttle.The engine note is apparently tailored to sound less like a diesel under stress and more like a sports car trumpeting the charge, but I'm not concentrating on the sound. I'm calculating how long I can keep up this headlong dash before having to rein it in for the more plebeian cars I'm rapidly catching up on.The acceleration is stirring as I'm pushed back into the leather. The gearchanges are almost imperceptible and the big car is holding the road and holding the curves without leaving me any room to doubt its ability.Smooth acceleration, smooth brakes, smooth gearshifts, this car is utterly unflappable and confidence inspiring. You have to check the dials to get any idea of the true pace you're travelling at. The perception of speed is so easily covered up in something as refined as this . . . officer.It's a sad fact that if you drive something as potent as this, you'll spend most of your life at the back of the queue of traffic. When you overtake to reach the front, you'll get a burst of activity before you reach the tail end of the next queue, and so on.While you're running in this car, though, it's impossible to avoid a wide smile. My racing colleagues at CARkeys may feel the suspension is a little soft, but that isn't my perception on the public road. Hitting the inevitable pits, pots and pimples on the face of the Queen's highway, left nothing more than a passing sensation of something being there. The curves were long, low and felt utterly without body roll.Like all Audi executive cars, the A6 saloon exudes sleek but understated elegance. There are slightly flared wheelarches, a twist of LED running lights around the headlamps, a sleek line up the bonnet, over the windscreen and roof that trails to the boot, and good sized wheels filling a sizeable percentage of the flanks.It also makes efficient use of the available space indoors. It's not big enough in the back seat to make it the ideal chauffeur's car, but there's enough room in there to make it a comfy ride for the length of the country. The self-opening boot is wide and deep with hooks and tie-down points and the option of folding the back seats forwards almost flat to create more space. Under its floor is a space-saver tyre and a few small storage areas.There are plenty of toys and gadgets in the test car's S line package, but if you need more there's an extensive list to choose from. As with other Audis, the prices of some upgrades are eyewateringly high (would you pay £920 for electrically adjustable seats, or £575 for an "advanced key"?) but it's up to you to decide what you need, want and can afford.In conclusion, the A6 BiTDI is a wolf in sheep's clothing, but with a vegetarian appetite. It's a tough contender that looks mild and unthreatening and merely sips at the fuel if it's driven without aggression. For all that, it's perhaps a little difficult to quickly form an emotional attachment to it. Apart from its level-headed efficiency, it's also one of the few cars on the road today which doesn't have a facial expression.It's a car-shaped car that's unpretentious and does exactly what it says on the tin - and a lot more besides. Engine 2967cc, 6 cylinders Power 309bhp Transmission 8-speed automatic Fuel/CO2 44.1mpg / 169g/km Acceleration 0-62mph: 5.1 seconds Top speed 155mph Price £46,835 Details correct at publication date