Freight traffic and functioning logistic systems are basic prerequisites for a successful economy. The combination of behavioral changes towards online shopping and the resulting need for high delivery flexibility will lead to a growing demand of light duty vehicles (LDV). Despite already existing models which predict the size of the LDV fleet based on a given transport demand, a dynamic connection between transport demand and vehicle fleet has not yet been realized. A distribution of annual mileages from LDVs, which have a wide range, leads to the assumption that fleet operators could adapt their mileage within set limits. This dynamic mileage adaptation is expressed as a module, which can then be integrated into existing fleet models. The results show that the number of LDVs needed to satisfy the demand is less volatile compared to the use of constant annual mileage. The results can certainly be seen as one step to more realistic fleet forecasting, yet further research in the field of fleet operator behavior would be beneficial.