... one answer is that motorists would not pay

According to Greenpeace, its Smile car would cost about 12 per cent more than a conventional Twingo - or around pounds 870 extra - if it went into mass production.

Would anyone buy it? If they were economists, driving the annual average mileage or more, they would - because the extra price would be covered by fuel savings in two years or less. If they were environmentalists who could justify owning a car at all, they certainly would.

The Smile (which Greenpeace originally wanted to call the Gringo) produces fewer smog pollutants, such as oxides of nitrogen and hydrocarbons. More importantly, it cuts emissions of carbon dioxide by over 40 per cent.

But what about the ordinary motorist? The car industry claims it knows its market inside out, and the public are not prepared to pay extra for such a car in an extremely competitive marketplace.

Industry experts also questioned whether drivers would like the feel and sound of a two-cylinder engine. But none of them doubted the technical achievements which Greenpeace Germany's deutschmarks have purchased from Swiss engine and car designers.

Europe's car manufacturers have promised national governments and the European Commission to cut the fuel consumption of the average new car by 10 per cent between the early 1990s and 2005. The SMILE goes way beyond that.

But it does not quite win the holy grail for the industry, which has been set at three litres to cover 100km. The German industry, by far Europe's largest, may begin manufacturing "micro cars", urban runabouts, which achieve that, in a few years' time.

But if such vehicles only fill a niche market, then they will not bring down the average fuel efficiency. The industry has been telling the EU Commission that an average of 5 litres per 100km for all new cars cannot be delivered by 2010.

Ever since the first oil price shock of 1973 manufacturers have been showing off concept cars with dramatically improved fuel efficiency. But, with very few exceptions, no prototype has come anywhere near mass production. In California, local laws make it mandatory for manufacturers to market a certain proportion of ultra-low emission vehicles. But the imposition of this law has been put back from 1998.