Audi A8 4.2 TDI

Drivers are already reaping the ben- efits of high-performance oil-burners, but it looks as if Audi's latest A8 TDI will raise the benchmark. The company claims its new flagship is the world's most powerful diesel production car - but is it torqueing the model up?

Replacing the 4.0 TDI, the 4.2-litre is 15kg lighter than its smaller predecessor. As well as the reduced weight, the unit uses the latest technology to inject fuel into the V8 at huge pressure.

The results are truly astonishing. Although it is the 321bhp power output that grabs the headlines - the 4.0 produced 50bhp less - the new V8 has incredible torque. The A8 delivers 650Nm to all four wheels between 1,600rpm and 3,500rpm. This wide band of thrust makes for effortless acceleration in any of the automatic gearbox's six ratios.

Use all the twin-turbo engine's performance, and 0-60mph takes only 5.9 seconds, with a top speed limited to 155mph. Amazingly, despite the supercar pace, Audi has also managed to improve economy and emissions.

The 4.2-litre returns a respectable 30mpg on the combined cycle, which is 0.6mpg more than the outgoing 4.0. What's more, its emissions comply with Euro IV regulations, and the 253g/km CO2 output puts the A8 in a competitive 19 per cent company car tax band.

From the wheel, the diesel demands few compromises. Refinement puts some petrol cars to shame, and occupants have to contend with minimal vibrations.

At ΂£58,605, the 4.2 TDI is no pricier than its predecessor, but ΂£10,240 more than the 3.0 diesel. The 4.2 petrol V8 is also less, at ΂£57,140, but the flagship oil-burner will save high-mileage drivers a fortune in fuel costs. With huge pace and frugal economy, the top A8 diesel again raises the question of why anyone would buy a petrol luxury saloon.