As regards road transport, comparisons between the fuels and engines used (or usable) are more complex. The two fuels most frequently used are gasoline and gas oil. The relative environmental effects of the two fuels are uncertain, Gas oil engines produce more particulate matter and sulphur dioxide but their consumption is lower: in particular, their contribution to the greenhouse effect is less (it appears that the balance between the specific consumption in litres per 100 km which is 15 to 30 per cent less for diesel is only partly compensated by the greater emission of CO2 per litre burned, which is of the order of 13 per cent). Selected results are illustrated in Table 2.13. Other possible fuels, some of which are already in use, are: