This paper derives a parametric equation for predicting the fuel economy of on-highway heavy-duty trucks. The accuracy of this equation is compared to actual vehicle MPG results. Based on this equation, the relative importance of the numerous parameters which affect the fuel economy of on-highway heavy-duty vehicles are calculated and discussed. To illustrate the value of this methodology, the development of the newest Freightliner EnergySaver package is described and the results from a cross-country fuel economy test of this package are presented. Using these same methods, the future for increasing on-highway heavy-duty truck fuel economy is explored.