SINCE the supplementary tax for Euro IV-compliant diesel passenger cars will be less punitive from July, several motor companies reckon people will start considering them when they next go shopping for a car.

Diesel cars emit less carbon dioxide than petrol cars, but let's not kid ourselves, global warming is probably not a huge concern for car buyers.

The main attractions of diesel cars? Higher torque and lower fuel bills.

To see if they live up to the reputation, we pit a Euro IV Citroen C5 HDI wagon against a bi-fuel C5, locally fitted with a CNG (compressed natural gas) kit.

Both cars are from Citroen agent Cycle & Carriage.

The HDI turns out to be far more driveable. Its 2-litre turbodiesel engine puts out 136 horsepower and, from 2,000rpm, a stonking 320Nm of torque.

The car is effortless on Singapore roads because of its immense pulling power, but petrol engines with forced induction will match it on this front.

At idling, the car's diesel chatter is audible - an inescapable trait of such cars. But it disappears once the C5 is on the go.

If you are lightfooted, the HDI will do 14km per litre. But because its engine is so energetic and the Citroen chassis is so well sorted, you are likely to drive it hard.

On average, it will give you at least 10km per litre, like the petrol-driven Volkswagen Jetta TSI. But because diesel is untaxed here, you will fork out less at the pumps.

However, you would have to clock more than 25,000km a year for the car to make economic sense. That's because its reduced supplementary tax still amounts to about $2,500 a year (on top of road tax).

The bi-fuel C5 is far less taxing on the wallet. Because CNG cars are given a sizeable tax rebate, this car costs about $92,000 - $7,000 less than the petrol-only C5. And because CNG costs less at the pump, the CNG C5's running cost should be 35 per cent lower than its petrol twin.

But you would have to fill up every two to three days because its range on gas is quite limited. And because there are at present only two CNG refuelling stations on the mainland (one in Mandai and the other in Jurong), this can be a chore.

Also, the test car's performance is severely diminished when running on gas. Take-off is especially sluggish, although the drop in power is less noticeable once the car gets to cruising speeds of around 80kmh.

Gear changes are also somehow affected. Downshifts especially are rough and loud.

A pity, since the C5 lends itself to CNG conversion. Its enormous boot is still roomy after accommodating the gas tank. And the car's self-levelling suspension automatically offsets the uneven weight distribution that CNG-converted cars suffer from.

The trick is to find a reliable conversion company, as the skills level and quality of equipment differ widely.

All said, the future of diesel and CNG cars is still hazy at this stage. Although gas is not taxed today, there is no guarantee that it will remain so. The price of diesel, on the other hand, has been rising more sharply than petrol.

In some countries, diesel costs more than 98-octane petrol. If the trend continues, and if petrol engines continue to improve in efficiency, the draw of diesel - or for that matter, CNG - will diminish.

This article was first published in The Straits Times on Mar 15, 2008.