An air-dam spoiler is commonly used to reduce aerodynamic drag in production vehicles. However, it inexplicably tends to show different performances between wind tunnel and coast-down tests. Neither the reason nor the mechanism has been clarified.We previously reported that an air-dam spoiler contributed to a change in the wake structure behind a vehicle. In this study, to clarify the mechanism, we investigated the coefficient of aerodynamic drag CD reduction effect, wake structure, and underflow under different boundary layer conditions by conducting wind tunnel tests with a rolling road system and constant speed on-road tests.We found that the air-dam spoiler changed the wake structure by deceleration of the underflow under stationary floor conditions. Accordingly, the base pressure was recovered by approximately 30% and, the CD value reduction effect was approximately 10%. The ratio of the base pressure recovery to the CD value reduction effect was approximately 90%, suggesting that the main mechanism is the base pressure recovery produced by changing the wake structure. In contrast, the air-dam spoiler did not change the wake structure in on- road tests and under moving floor conditions. Consequently, the base pressure recovery and CD value reduction effect decreased to approximately 20% and 4%, respectively, owing to the deceleration of the underflow being insufficient to change the wake structure. In other words, the boundary layer of the floor helped to decelerate the underflow and to change the wake structure under stationary floor conditions, suggesting that we overestimated the air-dam spoiler performance under stationary floor conditions.