A simple analytic relationship between fuel economy, vehicle parameters and driving cycle characteristics is established. Using publically available information on vehicle characteristics, the model can be used to predict fuel economy with an accuracy of about 5% (standard deviation). The model is based on two approximations: 1) an engine map approximation, and 2) an approximation for tractive energy. This paper emphasizes the second approximation, especially the energy consumed by the brakes. The model reveals the structure of fuel use in a way difficult to achieve with case-by-case numerical results and it enables study of fuel use in modified driving cycles.