This paper describes an eco-driving feedback system, OBDEnergy. Twenty-six drivers described their understanding of environmental impacts of driving before and after using OBDEnergy. Before OBDEnergy, participants discussed impacts in abstract, global terms (pollution, global warming). After OBDEnergy, participants appealed to concrete reference points (gallons of gas, trees required) with calculations and comparisons. We conclude that user-centered eco-driving feedback can contribute to pro-environmental behavior via increased awareness of the concrete environmental impacts of driving.