Peugeot and Citroen have combined to produce a diesel-electric hybrid engine.
The 1.6-litre unit, called the Hybride, was unveiled in the Peugeot 307 and Citroen C4.
Both cars are capable of an astonishing 82.9mpg with only 90g/km of carbon dioxide emissions.
A standard 1.6-litre turbodiesel 307 or C4 is capable of 60mpg and 125g/km emissions.
The Hybride system uses the existing models' diesel engine coupled to an electric motor. The electric motor also works as a generator to charge the battery when the car is decelerating.
Both cars can be driven as purely electric or diesel, or with a combination of both power sources.
Citroen and Peugeot say the Hybride technology could be on sale in production cars by 2010 but that costs make would make it prohibitively expensive at the moment.