BACK in the days when rickety sounding raspy diesel engines powered lorries, London taxis and little else it would have seemed inconceivable they would one day be refined and potent enough to power performance cars.

And though the idea of a diesel unit being fitted to a Ferrari or a Lamborghini might still be some way off the rise of the diesel has been nothing short of phenomenal.

And arguably the BMW X6 M 50d is the very pinnacle of that evolution to date.

It offers the sort of performance that even the purest petrol-head would covet and delivers it with a degree of frugality that makes it a no brainer when it comes to selecting it over petrol equivalents.

The idea of a big diesel SUV that can travel from a standing start to 62mph in 5.3 seconds is at first hard to take in but it's something you soon learn to love.

When the X6 was first unveiled I have to confess I didn't quite get the point.

Was it an SUV, a curious coupe, a crossover or something altogether new?

In reality it is a blend of all four, though my initial conclusion was it was some sort of curious dune buggy that no one would want to buy.

Like a lot of new creations that at first shock I have since warmed to it and am now at the stage where I do see the point.

SUVs are favoured for their styling, practicality, high driving position and four-wheel-drive and why should there be a rule book that states they should all stick to that box-like blueprint established by the original Range Rover.

In essence the X6 is an SUV with the body of a coupe stuck on top of it, albeit a large one.

It shares its underpinnings with the BMW X5 though with a considerable emphasis on the ‘sport' part of the SUV acronym rather than the ‘utility' bit.

Whether one thinks it a little too over the top is down to personal preference ultimately but it certainly has much to commend it.

My big worry was that coupe styling would severely limit is practicality and while it doesn't have the carrying capacity of an X5 it is still a very large family car.

This one was required to serve as a motor for a trip to France and fulfilled the task commendably.

A split level boot (one of the advantages of no spare wheel) meant it simply swallowed up large items of luggage, as well as those little bits one has to shove in too.

Even after loading everything up for a family of four for two weeks, including what appeared to be a large portion of Imelda Marcos's shoe collection, there was space to spare.

Fit and finish on the X6 is in line with the levels of quality and sophistication one has come to expect of the marque, with everything characterised by the hallmark of excellence.

The instrumentation is impressively intuitive and the once over complex iDrive system is now remarkably helpful and easy to use.

In truth there's little if anything to fault all round and comfort levels too are of the highest imaginable, complemented by a sublime ride reminiscent of class leaders like the Range Rover.

I could quite happily have been content at that, pootling along motorways at high speed, cosseted and cut off from the outside world.

But despite its undoubted luxury and comfort this X6 is also all about performance and once it's been sampled it's difficult to turn back.

Engage the Sport mode (after the first couple of times I left it on more or less permanently), familiarise yourself with the keen response of the throttle and you're ready to go.

Perhaps only something like a Porsche Cayenne at the higher end of the range or a supercharged Range Rover Sport could give it a run for its money but there's no doubting the driving experience is sheer bliss.

Given the vehicle's height and bulk it sits virtually flat when cornering at high speed, almost as if it's defying the laws of physics and the four-wheel-drive gives the kind of grip that helps ensure truly excellent handling.

Power is delivered seamlessly and smoothly too, thanks in large part to not one, or even two, but three turbos working together to get the most out of the 3.0-litre diesel unit.

The power never lets up throughout the rev range (a common diesel limitation) and the overall experience is rounded off by a responsive and well-weighted steering system.

If exploiting its prowess to the full economy levels obviously dip from a theoretically possible 36.7mpg figure but even then the M50d offers the sort of economy that's the envy of petrol-powered rivals.

Immensely powerful, yet impressively frugal it's hard to imagine a better blend - upon reflection I think I'm something of a fan of dune buggies after all.