Audi A6 BiTDI S line

The A6 BiTDI S line has a clumsy name but makes a great deal of sense. It mixes class-leading quality with style and practicality, while still being a seriously fast saloon. It’s quicker than any petrol A6 this side of the new S6, and although it’s more expensive than its key rivals, stronger resale values compensate.

The Audi A6 went straight to the top of the executive car class when it arrived in 2011, but can the all-wheel-drive S line flagship outmuscle the likes of the driver-orientated BMW 535d and Mercedes E350 Sport?

The A6 BiTDI S line packs a 309bhp 3.0-litre diesel V6 with two turbochargers and hooks it up to quattro four-wheel drive. It’s the ultimate diesel A6: the most powerful, most expensive and the only one to use Audi’s eight-speed tiptronic gearbox.

The S line package means the car rides 20mm lower than the SE, and brings beefier bumpers, side skirts and xenon headlamps with LED daytime running lights. At the back are LED tail-lamps and two chrome exhausts, while our car had 19-inch alloys instead of the standard 18-inch wheels.

The cabin is up to Audi’s usual high quality, and has lots of kit. There are electrically adjustable front sports seats as standard, plus a three-spoke S line steering wheel featuring shift paddles. There’s brushed aluminium on the door inserts and around the centre console, as well as the 6.5-inch display with sat-nav that’s standard on all A6s.

Push the start button and the diesel V6 emits a deep, bassy rumble, thanks to a resonator that’s been placed in the exhaust. It still sounds like a diesel, but it’s not rattly or harsh at all. Progress is smooth around town, although the tiptronic box still takes a while to react when you press the throttle at low speeds.

But once it’s past this, the engine really comes into its own. There’s a massive 650Nm of torque and gearshifts are smooth and swift. The car’s performance is effortless, while the sound is amazing, like a deep V8 warble that builds as the engine pulls the car along.

The BiTDI is only three-tenths slower than the new V8 turbo S6 from 0-62mph, posting a time of 5.1 seconds. Add strong brakes and it can mix it with some real sports cars.

There’s also Audi Drive Select. Leave it in Auto if you want to, but Comfort copes well with speed humps in town and makes the steering lighter for easy parking. Yet the body rolls more than when you select the firmest Dynamic mode; this sharpens up steering feel and throttle response for the most thrilling drive. Turn-in is instant, traction out of corners is excellent and the ride isn’t too intrusive, even in town.

The BiTDI is not only more potent than most petrol A6s, it’s also cleaner. It returns 44.1mpg and emits 169g/km of CO2.

At £46,160, it’s nearly £2,000 more than the 535d and £4,000 more than the (far less powerful) E350 Sport, yet will hold on to its value better. And this practical, high-quality performance saloon will eat motorways whole.