The aerodynamic performance of new vehicles is commonly determined using computational fluid dynamics (CFD) and wind tunnel tests. The final assessment is carried out by actual running tests. In particular, ideas regarding fuel consumption improvement that relate to components for the reduction of the coefficient of drag (CD) value are evaluated by coast-down tests. However, a difference often exists between the component's efficiency between wind tunnel tests and coast-down tests. Therefore, we focused on the efficiency of an air-dam spoiler in reducing CD values. A comparison was made between the aerodynamic effect of the air-dam spoiler in wind tunnel and coast-down tests in terms of the CD value and the wake structure behind the vehicle. To determine the relationship between the CD value and the wake structure behind the vehicle, we measured vehicle speed, wind velocity and direction, vehicle height, and pressure distribution on the back door. The results revealed that the CD values were reduced approximately 7% by attaching the air-dam spoiler in both the wind tunnel and coast-down tests. In contrast, the values of pressure coefficient restoration on the back door differed between the wind tunnel and coast-down tests. In the coast-down tests, the values of pressure coefficient restoration were approximately 16% less than those in the wind tunnel tests. Similarly, the Strouhal numbers of the pressure coefficient of the coast-down tests were smaller than those of the wind tunnel tests. This was considered to be causally related to the rolling wheels and the moving ground under the vehicle in the actual running tests. Given this, we should study the flow field behind the vehicle to elucidate the relationship between the CD value and the wake structure. This correlation also needs to be examined in terms of the pressure measurement results.