The van-type truck is frequently equipped with an air deflector to reduce fuel consumption. When studying the shape of the air deflector, it is necessary to choose a shape that is less sensitive to yaw characteristics. Side wind induced yaw conditions are more typical of on-road usage.In this study, the basic characteristics for a shape with strong yaw sensitivity were first examined. Subsequently, an angle change test was carried out by replacing the space over the cab with a rectangular parallelopiped and letting the upper and side edges, just in front of the rear body be the reference lines. The yaw angle was varied between 0°, 5°, and 10°. From this result, the existence of the optimum upper and side incident angles was confirmed. Furthermore, the reduction in fuel consumption using the optimum shape as compared with the conventional shape was estimated by simulation.