The results of previous Life Cycle Assessments indicate the ecological dominance of the vehicle's use phase compared to its production and recycling phase. Particularly the so-called weight-induced fuel saving coefficients point out the great spectrum (0.15 to 1.0 l/(100 kg · 100 km)) that affects the total result of the LCA significantly. The objective of this article, therefore, is to derive a physical based, i.e. scientific chargeable and practical approved, concept to determine the significant parameters of a vehicle's use phase for the Life Cycle Inventory.It turns out that - besides the aerodynamic and rolling resistance parameters and the efficiencies of the power train - the vehicle's weight, the rear axle's transmission ratio and the driven velocity profile have an important influence on a vehicle's fuel consumption. The coefficient for the reduction of fuel consumption on gasoline powered vehicles ranges from 0.34 to 0.48 ltr/(100 kg × 100 km) in the New European Driving Cycle (NEDC), while the saving on diesel vehicles is somewhat lower at 0.29 to 0.33 ltr/(100 kg × 100 km) in the NEDC.