An aerodynamic unit is configured for attachment onto a roof of a road vehicle, so that air passes over the unit during forward movement of the road vehicle. The unit has a first end for positioning distal to a rear end of a vehicle roof, and a second end for positioning proximal a rear end of a vehicle roof. The unit includes an inclined guide portion which decreases in height in a direction towards the trailing end of the unit. The unit also includes a flow director at or near the leading end of the aerodynamic unit, for directing air flow to the guide portion. The flow director is configured to have a height increasing in a direction from the leading end towards the trailing end. The unit may include vortex generators protruding from the guide portion. A fin may extend along each side of the unit in a direction between the trailing end and the leading end.