STUTTGART, Germany - Many drivers think that running a car on natural or fossil fuel instead of petrol or diesel will pay off in terms of lower fuel costs. According to a leading German car magazine however, the switchover only generates savings if a motorist covers at least 13 000km.Which, of course, most drivers do.There are two types of fuel available: compressed natural gas (CNG) or liquefied petroleum gas (LPG). Generally CNG is more expensive at the pumps than LPG although it burns more efficiently and is therefore more economical overall.ECO NOT SO CHEAP?Many cars can be adapted to run on LPG whereas most CNG-powered cars are designed and built from new to run on said fuel. The cost to convert to LPG varies widely between models.CNG cars are considerably more expensive than their petrol or diesel counterparts.Auto Strassenverkehr magazine calculated that in some cases a car owner would have to clock up a hefty 150 000km annually in a natual fuel-powered car to recoup the outlay.The magazine compared 16 LPG-converted and CNG models and assumed a yearly use of 15 000km and 30 000km for each. In each case the cost of conversion or loan finance in the case of the new cars was weighed up against the fuel savings.Of the LPG models, the Opel Zafira 1.4 LPG had the best results, allowing the driver to fully cover the extra costs from 13 000km a year on. The Dacia Logan MCV 1.6 MPI LPG starts to pay for itself from 19 000km and its Duster 1.6 sister from 20 000km.Among the more expensive CNG cars, the Zafira Tourer begins to recoup the higher outlay provided the driver covers at least 24 000km a year.