In this paper, we propose to make use of an auditory fuel efficiency display as means to support car drivers in adopting an energy-efficient driving style. We report on the development of the EcoSonic system as a platform for evaluating such displays and present five design approaches to guide their realization. In a study with 30 participants, we evaluated two prototype auditory displays against the baseline of visual-only eco-driving feedback in a within-subject study. Our selected designs are described in full detail. Key findings include a significantly reduced fuel consumption as well as lower engine speeds compared to the visual display. Furthermore, questionnaire analysis results confirm that the auditory conditions are less obtrusive and also seem to allow a more subconscious processing and comprehension of the provided information. Finally, we have found that the affectiveness of the display design seems to have a positive impact on its perceived helpfulness and the ability to absorb its information subconsciously.