A few years ago, the idea of a sports saloon with 376bhp that could cover 0-62mph in less than five seconds and return 45mpg would have been a fantasy. But the M550d xDrive does all that, as well as being great fun to drive in all conditions, without losing the 5 Series’ long-distance ability. Sadly, this engine won’t be seen in a UK 5 Series until the next generation arrives around 2016. Until then, the 535d M Sport will have to be enough.

This is the BMW M550d xDrive, which not only has a revised 3.0-litre six-cylinder engine with three turbochargers, but also four-wheel drive.

To all intents and purposes, it’s essentially a diesel BMW M5. It’s part of a new range of diesel M cars, which also includes X5 and X6 versions. But the revised engine – which powers all three – is the real star of the show. BMW has added a small extra turbocharger to its potent six-cylinder diesel.

Power goes up to 376bhp, making the M550d 63bhp more powerful than the 535d. Yet it’s the torque increase that really impresses – an extra 111Nm takes the peak figure to 740Nm.

BMW claims a 0-62mph time of only 4.5 seconds, although the incredible surge felt when you step on the accelerator makes that estimate seem conservative.

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Like all BMWs, the M550d’s top speed is electronically limited to 155mph, but we reached that number incredibly quickly on our test route, which included some derestricted autobahns.

The eight-speed gearbox has been tweaked to offer sportier responses. Combined with steering-wheel paddles, it makes accessing all that power easy.

The engine even makes a decent noise thanks to a sports exhaust: it sounds unlike any other diesel on the market.

So the M550d is certainly fast, but it’s also economical. When you turn the Drive Performance Control switch to Eco Pro mode, the eight-speed auto gearbox shifts up earlier and the air-conditioning is also adjusted to boost efficiency, so you should be able to get close to the claimed 44.8mpg fuel economy. What’s more, the car only emits 165g/km of CO2.

But with a much heavier engine in the nose, can the diesel M550d possibly be as good at tackling corners as the twin-turbo petrol M5?

Tipping the scales at 1,970kg, the newcomer only weighs 25kg more than the M5, which is even more impressive when you consider that figure includes the complex xDrive four-wheel-drive system, too.

BMW’s M Sport suspension is standard, although it stops short of a set-up as stiff and sporty as the M5’s. Still, the combination of adaptive dampers, all-wheel-drive traction and fast responses makes for a very enjoyable drive.

Select Sport or Sport+ modes and you have a car that sits in between the 535d and M5 in terms of performance. Choose Comfort mode, and you have a car that’s just as relaxing as any other diesel 5 Series riding on 19-inch alloy wheels.

At the moment, the biggest fly in the ointment is that BMW has no plans to bring the M550d to the UK. Sadly, re-engineering the car for right-hand drive presents significantly more problems than it does for the high-performance X5 M 50d and X6 M 50d SUVs.

BMW says this engine will come to the UK in the next-generation 5 Series instead.