IMPORTERS are experiencing a rise in sales of diesel cars that would seem to be wholly attributable to the steady increases in petrol prices.

According to importers like Citroen, sales of HDi versions of the C5 model are, perhaps for the first time ever, stronger than petrol. Being a Citroen, the actual volumes may not be very high, but the leaning towards diesel does indicate a growing awareness among Australian car-buyers of the merits of combustion ignition.

Diesels, almost without exception, are a lot more frugal than petrol engines. Only the petrol-electric hybrids such as the Toyota Prius and Honda Civic hybrid quote similarly thrifty fuel consumption figures.

Diesel, to many, has always made sense. But in today’s climate it makes even more because, in addition to the much better economy, actual pump prices also tend to be lower than petrol.

And, in the case of Citroen’s medium-sized C5, there’s also the fact that there’s no penalty to pay as far as retail price of the vehicle is concerned. Diesel and petrol C5s are virtually equivalent in price.

There are the perceived downsides, of course. There’s the noise, for one. And in the past there was always something of a performance deficit to be paid.

Turbocharging has generally taken care of the latter, to the extent that some turbo-diesels are sometimes at the top of the performance tree. Audi’s new A3 turbo-diesel, for example, is the fastest model in the range, while VW’s V10 turbo-diesel Touareg is a stunning testament to the fact that diesel expectations are changing.

Citroen’s C5 turbo-diesel tends to belong more to the old school. The 2.0-litre engine, which is shared with Peugeot in that company’s 406 HDi, is a producer of torque rather than outright power, more truck like in its characteristics than highly strung.

But there’s something endearing about the C5 HDi that may have everything to do with the fact that it spends a lot more time on the road and out of filling stations than most cars. It is a standard expectation that it will return better than 8.0 litres per 100km in any circumstances, and way better than that again on the open road.

Yet at the same time it is a car that needs to be treated by the driver with a special level of understanding. Things you assume in a regular, reasonably powerful petrol-engined car aren’t necessarily going to materialise.

A passing manoeuvre on the open road will need a little bit of forethought, while a quick getaway from the lights can only be partly achieved, and then only by being really brutal with the accelerator foot.

The friendly, chuffing, diesel noise is always there, although once the C5 is under way it is hardly noticeable. It’s usually when the car is idling in an enclosed space, where the noise bounces back, that it becomes really evident.

Comparing the Citroen engine with many diesel newcomers will provide a clue as to why it’s not as powerful, in terms of specific output, as most. The configuration is very straight-forward cast-iron block, single-camshaft eight-valve cylinder head.

The outputs are similarly bland, with its 82kW produced at a very conservative 4000rpm, and a hefty-for-capacity but hardly groundbreaking 255Nm of torque.

The upside is that this comes in at 1750rpm, which means the torque is on tap virtually from the moment engine rpm rises above idle.

The low revving character is the reason for the necessary driving-habit change, particularly with the manual transmission, which gives its best if the driver has the confidence to rely on torque, rather than revs.

The four-speed automatic transmission, complete with sequential mode, is really a better proposition.

That said, this is not the best of autos. It has a tendency to hold tenaciously onto a high gear way past the point where anything else would have shifted down a ratio, while on the other hand it will downshift without intervention from the driver on a downhill slope, which is usually a good thing.

The problem is that it does this almost indiscriminately, usually defeating its basic purpose which is to avoid the frustrating "hunting" from ratio to ratio on hilly terrain. It’s almost British in that it appears simply to be trying to be too clever.

There’s a sport mode too, but this tends to hold lower gears to the extent that fuel economy would be compromised. And some of the shifts are accompanied by an uncomfortable thump somewhere in the system.

At the end of the day, though, there’s always the sequential manual override to fall back on.

The HDi’s fluid suspension system is the third iteration of Citroen’s Hydractive system and is a simplified, low maintenance and more feature loaded system than before.

The system uses a central hydraulic fluid reservoir that feeds four suspension spheres – one for each wheel – that operate as the springing medium and also allow variations in ride height.

The difference in the new system is the adoption of two extra spheres – one front and back – that also vary the overall stiffness of the system.

Turbo-diesel C5s do miss out, however, on the selectable sport/comfort modes that allow the driver to choose between two different types of ride/handling characteristics in petrol versions.

The system also achieves a certain degree of "active" suspension control via the electronics. In order to allow the system to react quickly to changing circumstances, it measures variations in ride and height and suspension movement, steering wheel angle and the speed at which it is turning, activity at the accelerator pedal, engine speed, brake pedal pressure and road speed.

The suspension comprises MacPherson struts at the front and an aluminium cross beam at the back with cast-iron trailing arms and self-steering geometry.

The good thing is that all this works, giving the C5 a significant point of difference compared with its main competitors.

The ride is very good, very absorbent, yet the electronics allow the biggish wagon to sweep through tight corners with very little bodyroll and quite a degree of steering accuracy.

But there is a sign of some uncertainty on high-speed bends where the system needs to balance between adequately absorbing the long undulations and maintaining an even stance.

Other handy aspects of the Hydractive system are that it can be used to minimise wheel-changing exertions (a button on the centre console is used to lift the car to its maximum height, then used to retract the offending wheel so it can be removed after installation of the jack) and, at speeds above 110km/h which are only legal in the Northern Territory, it automatically lowers itself by 15mm at the front and 11mm at the rear to maximise balance and aerodynamic efficiency.

But if this feature is rarely going to be appreciated in Australia, the C5’s ability to maintain a steady ride height regardless of load surely will be - as also will the ability to run at an increased ride height over dodgy, pot-holed roads in the bush.

The brakes are pretty good too, claiming all the appropriate technology including discs front and rear, anti-lock, electronic brake-force distribution and brake assist to add boost in emergency situations.

The HDi’s steering is nicer somehow than that of the road-speed sensitive system used in V6 C5s. Whereas the petrol-engined car’s system tends to feel slightly artificial in terms of rim loads, the HDi’s is more linear and less disturbing.

As a wagon, the C5 works very well. It’s not Commodore-size (more like a Peugeot 406), yet it is actually quite an imposing presence.

The cargo area, particularly with the split-fold rear seats double-flipped into maximum load position, is large and relatively undisturbed by unwanted intrusions.

A fully-assembled mountain bike can be inserted easily and there’s a button just inside the right-hand side of the rear door that can be pressed to drop the back end down to its lowest setting for easy access. Once the tailgate is closed, the C5 rises back to its regular ride height.

A two-position anti-intrusion cargo net (either behind the rear seat or the front seats, depending on how the cargo area is configured at the time) is a good feature that should be found on all wagons, although the retaining hooks in the roof are a tad fiddly to operate.

But the C5 Estate is certainly one of the better wagons, offering most of the required utility, yet being a comfortable and quite handy driver’s car into the bargain.

In HDi form it throws exceptional fuel economy into the mix.

Today, and tomorrow, it’s an almost certain bet that new-car customers will be including that factor more and more prominently in their wishlists.