[Piwoslaw]

I found a presentation by the ACEA (European Automobile Manufacturers Association) on improving commercial vehicle efficiency and reducing their emissions:
COMMERCIAL VEHICLES, FUEL EFFICIENCY AND CO2, Challenges & Possible solutions
On page 4 it suggests that fuel efficiency for cargo hauling vehicles shouldn't be measured in liters per 100km as this is very misleading. A much better unit would be liters per 1000tonkm, which factors in the amount of cargo that can be carried. For large-volume cargo and passengers, m3-km and pass-km should be used, respectively.
Also, the testing procedure should be changed to better fit different types of duty cycles for different types of trucks.

Also, slide #7 shows that current European long-haul trucks already produce 30% less CO2 per ton-km than EPA's 2010 baseline for Heavy-Duty Vehicles (42 vs 61 gCO2/tkm @ 75% utilisation of loading capacity), and are still better than the US 2017 target (49 gCO2/tkm).