“…In the first case, results are usually expressed per kilometer driven by a specific vehicle (e.g. Edwards et al, 2008;Hekkert et al, 2005;Lechón et al, 2005Lechón et al, , 2006Lechón et al, , 2009, also per MJ consumed in the engine (Ou et al, 2009). In the second, impacts are usually expressed per unit of fuel produced, either kg, MJ, etc.…”