To date the effects of winds on the computed fuel economy of highway vehicles have been omitted or included in simple ways. This paper presents the results of a rigorous computer study on the realistic effects of winds upon the aerodynamic resistance, and consequently, upon the fuel consumption of automobiles for EPA driving cycles. The aerodynamic resistance was a function of the instantaneous wind direction and speed relative to the car. A wind spectrum having an annual speed of 10 mph will increase the zero-yaw aerodynamic resistance by 16% for the Combined driving cycle. Several simplistic approaches were evaluated against rigorously obtained results.