Hybrid electric vehicles (HEVs) have the potential to accomplish high energy efficiency (i.e., low fuel consumption) given that drivers apply effective ecodriving control strategies (i.e., ecodriving behavior). However, HEVs have a relatively complex powertrain and therefore require a considerable knowledge acquisition process to enable optimal ecodriving behavior. The objective of the present research was to examine the acquisition of ecodriving strategy knowledge in HEV drivers who are successful in achieving a relatively high energy efficiency. To this end, we recruited 39 HEV drivers with above-average fuel efficiencies and collected interview data on the ecodriving strategy acquisition process. Drivers reported the acquisition of different types of knowledge as important for ecodriving, namely specific strategy knowledge and general technical system knowledge. They acquired this knowledge both with system-interaction (e.g., actively testing specific strategies, continuous monitoring of energy consumption) and without system-interaction (e.g., internet forums, consulting experts). This learning process took drivers on average 6.4 months or 10062 km. The results show the high diversity of the means that HEV drivers use to develop their ecodriving knowledge and the considerable time it takes HEV drivers to develop their ecodriving strategies.