By Christopher Tan
Senior Correspondent

It has a 2-litre four-cylinder engine which offers almost the performance of a 2.5-litre six-cylinder - while using a lot less fuel.

But unless you drive 27,000km or more a year, the BMW 520d, a turbodiesel 5-series being tested over two weeks here, will not make much monetary sense.

That is because the 520d, like all Euro IV diesel cars here, attracts a special tax of S$1.25 per cc of displacement on top of its normal road tax.

If you clock an annual mileage of 27,000km, a year's worth of diesel, plus road tax and special tax, add up to just over S$7,000.

A petrol-driven 525i will chalk up more than double the cost in fuel (based on the highest grade of petrol here). Together with road tax, its annual running cost will be around S$8,700.

That way, the 520d's annual running cost will be close to 20 per cent lower than the 525i's.

But, if like most motorists, you clock 20,000km or so a year, and you use 95-octane, the equation will be different.

In short, the 520d's cost advantage will narrow to less than 10 per cent.

Why compare the 520d to a 525i?

Because the 520i, the petrol 5-series with the same displacement, pales in comparison when it comes to performance.

The turbodiesel has more peak power, more torque across the rev range and better acceleration all round.

On paper, the 525i is superior. It clocks a 7.9-second 0-100kmh sprint and takes 28.1 seconds to cover 1km. The 520d takes 8.4 and 29.2 seconds respectively.

That is because the petrol car puts out 218bhp versus the diesel's 177bhp. But in day-to-day driving in a place like Singapore, the 520d's 350Nm of torque - available from 1,750rpm - makes the car far more driveable than the 525i. The latter has 250Nm from 2,750rpm.

Indeed, the 520d's responsiveness is a joy to behold, whether you are tooling about in town or overtaking on the highway.

The turbodiesel characteristics suit the Beemer DNA to a T. There is none of the lag or flat spots you sometimes encounter with the marque's normally aspirated petrol models.