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: