Wind tunnel experiments have been conducted to study the transient aerodynamic forces experienced by the members of a four-car platoon during passing maneuvers with a single vehicle. The drag and side force coefficients on each of the four platoon members are measured simultaneously using strain gauge balances during the maneuver to characterize the transient effects. The results show that every car in the platoon experiences significant changes of drag and side force coefficients when the passing model moves to the proximity of each car. Influences of the lateral spacing and relative velocity between the platoon and the passing model, as well as the shape of the passing model, have also been investigated. Similar trends in force coefficients are observed in both simulation of a vehicle passing a platoon and a platoon overtaking a vehicle.