An aerodynamic device is arranged for mounting to a vehicle or a trailer for reducing its wind resistance. The aerodynamic device includes at least one sheet material arranged to be mounted to the vehicle/trailer and associated with a corresponding wall portion of the vehicle/trailer. Each wall portion has an outer surface and an inner surface and extends towards the rear of the vehicle/trailer. In a deployed first state, each sheet material is arranged to at least partly extend beyond a rearmost position of the wall portion. In a stored second state, each sheet material is arranged to be inserted such that is faces towards the inner surface. A vehicle or a trailer including the aerodynamic device is also provided.