How does 85.6mpg and 88g/km of CO2 sound? Meet the world's most economical production car.

Its emissions are a full 10g/km less than the nearest CO2 emitter, the Ford Fiesta Econetic, and its fuel consumption eclipses everything else on the road.

Stuart Milne tested it on the roads around Mercedes-Benz World in Brooklands, Surrey.

Smart is making the most of being one of the first car makers to sell genuinely fuel efficient cars in Britain. It has brought out Stop/Start features on some of its petrol models which are known as the Smart Fortwo MHD, and has a fully electric model waiting in the wings.

But one of the biggest stories of 2009 will be the Smart Fortwo CDI.

It’s a car that's been on sale in Europe for several years, but will launch on these shores on March 1, 2009.

Under the bonnet sits the world's smallest direct-injection diesel engine, which borrows technology from far larger engines in the Mercedes-Benz stable.

At the heart of this technology is two-phase fuel injection which injects a small amount of diesel into the engine's combustion chambers, preheating the cylinders for reduced noise.

Smart engineers have employed three stages of emission control. The common-rail diesel engine is efficient in the first instance, but what emissions are produced are reduced further with up to 60 per cent of gases being fed back into the engine to be burned again. This lowers harmful nitrogen oxides which would otherwise be emitted.

A traditional catalytic converter treats the majority of remaining gases within the exhaust.

It also employs turbocharging for improved performance, and while it might only develop 45bhp from its 799cc (0.8-litre) engine, performance isn't far shy of its petrol-powered siblings.

It will reach 62mph in 19.8 seconds, before reaching 85mph; but like most diesels these figures only tell half the story. It has 81lb/ft of pulling power, and feels as though it has the edge on the petrol models when overtaking, but like other Smarts, the gearbox can be slow to shift gear.

The trade-off for the diesel engine is refinement. There's a great deal of noise and vibration through the cabin, which never really goes away, so it should be worth investing in the £200 Smart Sound System which offers extra speakers – including a subwoofer – and MP3 connectivity.

But for today's buyer, there are things more important than a bit of noise. Despite the Fortwo CDI's shortcomings, its impossible to look past those fuel consumption and CO2 figures.