Not often do we get a chance to test three of the finest medium-sized diesels on the globe at the same time. Not only are they the most dulcet drives around, they're also so way out in dynamic terms, they take on hotshot robot-racing hatches prowling our streets. And when you add other bonuses such as great road holding, comfort, genteel ride and engine silence, they fit any aficionado's Christmas shopping list.

This type of car not only surprises the enthusiast but also the pastor cruising to the awaiting flock with piped undertones of the Gregorian chant filtering through the still cabin. But more than anything else, it's perhaps the manner in which the three cars here achieve silent and effortless travel without bothering too much about what gear you're in.

Torque and resilience are kings here Diesels were never silent - neither were they smokeless. And when you wanted some action by depressing the accelerator, not much happened. In fact, it would take time to achieve a modicum of action, much noise and little else.

Try this with the Audi 3.0 TDI, BMW 330d or the Mercedes-Benz C320 CDI, and you're in for the experience of your life. As you floor the accelerator, an uncanny and silent surge squeezes you back into your seat from around a low 2000rpm, with little else but the speedo indicating your progress. In fact, the experience is most gratifying in the Merc with its refined 7G-tronic 'box - and when you reach the indicated 250km/h, you're stunned at the manner in which it was attained. To this end, it should be remembered a petrol-driven engine achieving the 100km/h in a fraction under seven seconds, does this to the exciting accompaniment of engine noise and a rev counter dancing backwards and forwards between 5000 and 7000rpm between gear changes. With the Merc it's as though a magic wand is responsible for the surprising results.

However, not for one solitary moment should you believe this is not possible in the either the Audi or BMW with automatic 'boxes. They would achieve similar results. And that is where diesel technology is going. To this end, cast your mind back a few weeks, to remind you what the Audi R10 sports racing car achieved with its automatic gearbox. It won the Le Mans 24 Hour - again with an Audi diesel. But if the three cars here may achieve the same feel when driving, the performance results point mainly in favour of the Merc - this with an automatic gearbox. To be kept in mind is automatic 'boxes in petrol-driven performance cars do not offer the best results due to their high revving nature and wide rev ranges. With 'hot' turbo-diesels it's another story.

The rev ranges are narrower, torque higher, causing more gear changes to achieve the same speed goals.

As stated, it's quite something watching the speedo swing across the dial in almost seamless silence with the 7G-tronic 'box. There is, however, a downside to the automatic box with its unwanted launch lag, which, when added to the turbo lag, can add to some dangerous cliffhanging moments when you decide to nip across traffic lanes. With a manual box you can control your take-off, and even add to your chances, by slipping the clutch to maintain safe closing traffic distances. Instead with a turbo/automatic car, such as the C320 CDI, it takes almost 2.5 seconds for the two devices to find one another. Result? Once you've committed yourself, you're left to the vagaries of blaspheming drivers bearing down upon you - with nothing else but to hope for the best!

All three have the road holding and brakes to take good care of you. But the BMW still leads the way when it comes to nimbleness and response, even though the Audi A4 and C are getting closer. It was BMW which first pioneered lightweight suspension and is still refining it, and when added to the rear wheel drive set-up, goes a long way in attaining refined, responsive handling. The Merc has the same rear wheel drive layout, while the A4 Audi makes use of its AWD quattro set-up. The common denominator here is that none are front wheel driven - a definite negative for refined road holding when driving at extremes.

As far as the Audi is concerned, the quattro set-up is a refined addition - but adds weight to the front of the car, resulting in a more uneven weight distribution than either the BMW or Merc. But then the quattro set-up scores when road conditions are slippery, and the reason it does not need much of the safety gadgetry required by its two rivals here.

When it comes to engine technology, the A4 and Merc have state-of-the-art V6 quadcam engines, while the BMW still makes use of its evergreen in-line-6 twin-cam mill. Current engine philosophy is against the use of an in-line-6, due mainly to its longer and therefore more flexible crankshaft. However, in terms of its Valvetronic valve system the BMW is ahead in engine breathing technology.

Then comes the matter of fuel consumption. Again one should remember that with the fine performance figures attained, better than similar sized petrol siblings, the three are true eye-openers in these terms. Between 15 and 20% lower than the petrol siblings, they do attract Mr and Mrs Everyday. And should you decide to travel north of our borders, with their petrol from the Stone age, you score handsomely with readily available diesel - and your petrol stops are also more controllable.

The cabin of the BMW is more exciting than those of its two rivals, which are as well finished, but do little to attract a second glance - with the Audi perhaps having slightly more flair than the Merc. And when it comes to pimping your ride, the three have packages to personalise.

But the die has been cast. Relative to diesel-powered cars, the petrol versions have suffered in terms of market penetration. With cars of the calibre of the Audi A4 3.0 TDI, BMW 330d and Merc C320 CDI buyers will more come realise that though you pay a premium for diesel powered versions, the amount of fuel you save in fuel terms, goes a long way in making up the difference.

However, it's a close thing when making a choice. At R370 000 the Merc C320 CDI seems to justify its premium when you take its performance abilities into account, while the BMW 330d's R361000 is seemingly a better buy than the R359 000 for the Audi A4 3.0TDI Quattro - for the same reason.