When comparing biofuels with fossil fuels, it is of utmost importance to consider the same relevant service from the various systems. In case of motor fuels, as long as mobility is concerned, this service must be related to mechanical energy, in other words, to the distance travelled. Most studies however choose 1 MJth of fuel (in the tank) as the functional unit, regardless of the type of fuel (ADEME-DIREM-PWC, 2002; Elsayed et al., 2003; EMPA, 2007a; GM-LBST, 2002; Malca and Freire, 2006; Shapouri et al., 2002). This choice is not rele­vant as the mechanical efficiency can vary from one fuel or engine to the other.

For example, several tests (AEAT, 2002; EMPA, 2007b; IDIADA, 2003) have shown that the consumption of E5 (fuel blend consisting of 5% vol. bioethanol and 95% vol. gasoline) in liters is slightly less than the consumption of gasoline for the same service, that is, 100 km. In this specific case, it means that 1 MJth gasoline should be compared with less than 1 MJth E5. For simplicity, if one considers that the consumption of gasoline (in liters per 100 km) equals that of E5, then, 1 liter of fuel (gasoline or ethanol) should be a good functional unit for comparing ethanol with gasoline when the blend is E5. Using (even for simplicity) 1 MJth of fuel as the functional unit, when comparing gasoline to ethanol, means that one makes the implicit choice that 1 liter of gasoline should be compared with 1.5 liter of ethanol (given as the ratio of the LHV of gasoline, i. e., 31.9 MJth/l, to the LHV of bioethanol, i. e., 21.2 MJth/l). This choice between liter and MJth would be relevant however if ethanol were used as pure fuel (or at least as the main component of the fuel blend, e. g., E85) or in the case of heat applications.

ADEME (2010) has considered the effect of biofuels incorporation rate on the vehicle/fuel performance (in terms of liters per 100 km or MJfuei/km).