Things are not always as they seem. If a car looks like a sporty V8, goes like a sporty V8 and, most importantly sounds like a sporty V8, surely it can only be one thing, right? Apparently not. The latest version of Audi’s sleek A6 sedan is set to shatter a few expectations.

Despite ticking all of the boxes above, it’s not a V8 at all. It’s only a V6. And a diesel V6 at that. The great deception of this car is an engine note that tricks you into thinking it’s something that it’s not. Not to mention performance that’s the equal of just about any high-output V8, matched with fuel-efficiency, which would put some hybrids to shame. That’s quite a combination.

Audi has built a reputation for shattering expectations – particularly around diesel engines – in recent years. The German brand has won the famous Le Mans 24-hour race using diesel-powered machines to out-muscle, and then out-last, an armada of the world’s best sports car makers.

Until then, the notion of a diesel engine in a performance machine was rather far-fetched. No longer.

Audi even refers to this latest powerplant as a “performance diesel”. That’s probably a bit like “delicious diet food” but while it might once have been a contradiction in terms, it’s now the new standard at the pointy end of the automotive spectrum.

At the heart of this new A6 is the German manufacturer’s latest 3-litre, biturbo diesel V6, pumping out numbers that almost defy physics. It boasts 230 kilowatts and a gargantuan 660 Newtonmetres of torque – dwarfing the pulling power of most engines. Yet all this grunt results in a combined average thirst of just 6.4L/100km – or just over 5L/100km when the car is cruising on the highway.

In practical terms, it will rocket the A6 to the speed limit in a tick over five seconds – comparable acceleration to a Holden HSV or Ford FPV V8. Audi says it’s the fastest diesel production machine in Australia. Yet its consumption is more in line with a four-cylinder engine – and a rather frugal one at that.

At highway cruising speed and in eighth gear, the engine is barely ticking over at 1200rpm, helping deliver that remarkably frugal thirst – further helped by a stop-start system that kicks in when the car is stationary. But perhaps most surprising of all is the way it sounds. That’s thanks to a deceptive piece of technology called the sound actuator, effectively a loudspeaker that pipes the V8 soundtrack into the car’s exhaust system. What’s more, this rorty sound system is even adjustable.

When we got into the A6, its engine seemed to rumble and grumble like a V8 Supercar. But a click of the dial on the car’s central management system allowed us to select “Comfort” mode, quietening this rumble to a vague burble. You can even mix and match the volume with different settings for throttle response and the car’s suspension set up.

Want the engine and transmission to be more dynamic, but the exhaust note quieter? Click. Want to firm up the suspension, but set the transmission to comfort mode? Click. It truly does allow you to buy one car, yet have several different cars to drive.

And make no mistake – in Dynamic mode this car is remarkably good fun. It’s as responsive and punchy as any V8 – perhaps more so because of its massive torque band – yet can be dialed down to a smooth, urbane luxury sedan when the family’s on board.

For the technically minded, the new biturbo V6 uses sequential turbo-chargers – one smaller, variable-turbine version, which does most of the work at low-speeds and a larger turbo that takes over from about upwards. Unusually for a diesel, it will happily spin to 5200rpm.

In our test machine, all that driveline technology is packed into a car that, in itself, is one of the most improved models in its class.

The A6 has always struggled to find traction against its German arch-rivals, the BMW 5 Series and Mercedes-Benz E-Class, as well as other contenders such as Lexus’ smart new GS and the beautiful Jaguar XF. But with its Quattro all-wheel-drive system and a vastly-improved interior, particularly the clever instrument panel treatment and superb cockpit finishes, the A6 is fast making ground on its rivals.

Add to that the exemplary road manners of its adjustable suspension system and the A6’s $118,000 price tag starts to look like quite a bargain. Helping that value equation is a thirst well below 7L/100km.

Standard kit is generous, too – including rear-view camera, handsome big alloys, a superb Bose audio system with Bluetooth streaming; keyless entry, electric memory seats, sunroof, Xenon headlights with LED driving lights and a high-end navigation and management system.

Audi says that diesels already account for more than half of its A6 sales and this new biturbo seems likely to enhance that appeal.

Always a good car, the A6 felt just short of being a great one in such esteemed company. But I suspect that, particularly with this engine package at a tasty price, it’s going to attract a whole lot more attention.

And not just because of the way it sounds.

AUDI A6 3.0 BITURBO

DETAILS: Four-door, five-seat full-sized luxury sedan with dual turbo-charged diesel engine and eight-speed automatic transmission.

TECH STUFF: 3.0-litre, common rail direct injected 2-stage turbo-diesel produces 230kW@3900-4500rpm, 650Nm@1450-2800rpm; eight-speed tiptronic transmission with sport mode and wheel-mounted gear-change paddles; Quattro permanent all-wheel-drive system.

FEATURES: Front, side, seat-mounted, rear and curtain airbags (10 in all); electronic stability control with anti-slip, ABS and ABC with brake assist; electromechanical park brake, Milano leather upholstery, electric seats, windows and mirrors, keyless entry; satellite navigation, adjustable suspension, automatic wipers and Bi-xenon headlights; Bose audio system with Bluetooth streaming.

THIRST: 6.4L/100km.

VERDICT: A diesel in name only.