THERE are one or two negatives when it comes to comparing the running costs of a four-wheel drive wagon with a regular sedan car.

When you consider all 4WDs - whether they be heavy-duty, light-duty of "soft-road" - are inclined more towards light truck than passenger car specifications, it’s understandable that the costs of replacing items like tyres, brake pads and the like could be expected to be more expensive than your average Holden Commodore.

Then there are the fuel costs. No 4WD, with its combination of weight, physical size and hardly-aerodynamic shape, could even pretend to have a head start on fuel economy. Two tonnes of perambulating metal is always going to demand more than an average amount of energy to get it moving.

That’s why turbo-diesel engines make a lot of sense in such applications. Diesels because they generally consume less fuel than petrol engines turbo-diesels because they offer similar performance.

In fact, so good are today’s turbodiesels that many 4WDs are actually able to better the fuel economy of some smaller, lighter petrol-engined cars.

That’s why so many buyers opt for turbodiesel Toyota LandCruisers, Nissan Patrols or Mitsubishi Pajeros. Drastically reduced fuel costs, as well as increased cruising range, are considered worth the extra money traditionally asked for diesel power – although, in a growing number of cases, the surcharge for diesel engines is reducing, sometimes to the point of price equivalence with petrol alternatives.

And today’s diesels don’t just offer a combination of fuel economy and strong performance many modern turbodiesels are almost as quiet and smooth running as petrol engines – particularly from inside the cabin.

BMW’s latest version of its X5 – the 3.0d turbodiesel - is perhaps the best example available. The 3.0-litre inline six-cylinder engine uses a variable geometry turbocharger, a four-valves-per-cylinder layout and common-rail fuel injection.

It only came to Australia after consistently higher-quality diesel fuel became available here from early 2003. Unlike diesels of the past, there’s no need to wait for the glow plugs to warm up before starting: the turbo-diesel fires instantly in temperatures as low as five degrees.

In the style of most turbo-diesels, it offers a torque output that overshadows the already more than acceptable power figures. While the X5’s 135kW would be considered quite reasonable for a similar-size, non-turbo petrol engine, it’s the 390Nm of torque – from 2000 to 3000rpm – that really impresses.

This is more akin to what you’d expect of four or more litres of high-tech petrol engine, and then at much higher revs.

The fuel consumption figures make an even stronger case the two tonnes-plus turbo-diesel X5 actually manages to be more economical than a V6 Commodore, with official figures quoting 7.1 litres/100km on the highway and 10.0 litres/100km in urban conditions.

The five-speed manual version is even better at 6.4 and 9.3 litres/100km. Put into context, this sort of economy allows something like 1000km between 93-litre tankfuls.

If there’s a downside to all this, it’s difficult to identify.

The X5 turbo-diesel, from the driver’s seat, sounds and feels so much like a petrol engine that only the truly critical driver would notice it, while the torque delivery is strong enough to imbue it with impressive, easy acceleration.

BMW claims the manual version will reach 100km/h in 10.8 seconds in automatic form (slightly quicker in manual), which is hardly tardy, and only about two seconds away from 4.4-litre petrol V8 versions.

The fact that the same engine is also fitted to BMW 3, 5 and 7 Series models in Europe indicates how far along the path of refinement diesel engines have come.

Combine this with the X5’s general reputation for class-leading on-road abilities and the turbo-diesel BMW remains a 4WD with a distinct sporting edge.

Stepping out of an X5 into any other soft-roader currently on the market, you’ll be immediately impressed by its ability to please the driver.

There’s a sharpness to the steering, a smooth, linear response on corners, that tends to confirm BMW’s claims it has been engineered to handle more like a car than a top-heavy 4WD.

The turbo-diesel is well equipped too, although it probably loses ground here if compared on a feature-for-feature basis on recent soft-road newcomers like the Lexus RX330 and Honda MDX – or even Volvo’s new XC90.

Externally, it looks pretty much the same as the 3.0-litre petrol X5. Both are fitted with 17-inch alloy wheels, fog lamps, roof rails and a split rear tailgate.

Inside, it gets ten airbags, leather upholstery (with a leather cladding on the steering wheel, handbrake and gearlever), climate control air-conditioning, in-dash CD player with six-CD stacker and a trip computer.

However, because it’s a BMW and doesn’t concentrate so heavily on value-for-money, the X5 3.0d misses out on things like the sunroof you get in a Honda MDX, or the GPS navigation system offered in the Lexus RX330. And the power-adjusted front seats that are standard in both Japanese vehicles.

But the X5 basics are there.

The X5 uses a three-differential drive system that sends 62 per cent of power to the back wheels and a front differential arrangement that sends one of the axle shafts through the sump to keep weight low and allow equal-length drive shafts.

There is no dual-range gearbox, but a host of electronic systems help keep things in hand when venturing off road – something the X5 is up to, if only on an occasional basis.

An automatic differential brake ensures that power is sent where it’s needed, while steep descents are taken care of by BMW’s hill-descent control that works, much like a Range Rover, by automatically applying the brakes at low speed to maintain an appropriate crawling pace without need of any intervention from the driver.

As expected, the X5 turbo-diesel also gets the standard stability control system, plus BMW’s cornering brake control as well as a four-channel anti-lock braking system that is claimed to give equivalent performance to BMW’s 7 Series sedan.

The five-seat interior is wide and generous of legroom, as well as being relatively easily accessed – for a 4WD. It’s the sort of vehicle into which you can throw a mountain bike or two, via the twin tailgate and 60-40 split-fold rear seats, without too much hefting and juggling.

The arrival of new soft-road 4WDs such as the Lexus RX330, Honda MDX, Volvo XC90, Volkswagen Touareg – and even the stratospherically priced Porsche Cayenne – means the market has become increasingly crowded, but the BMW X5 still remains as something of a benchmark.