A road vehicle 10 including a tractor 12 for pulling a trailer 14, and a trailer arranged to be pulled by the tractor where the trailer has a cab unit 16 and a cab top adjustable fairing 50. The adjustable fairing includes a deflector moveable between a raised position (figure 2) and a lowered position, an adjustment mechanism (60 figure 3) for moving the deflector between said raised and lowered positions, a control system for controlling the adjustment mechanism, and a pressure sensor 56 for determining the air speed in front of the adjustable fairing. Where the control system monitors signals from the pressure sensor, and determines whether to adjust the position of the deflector. A height sensor (20) may also be provided so that the height of the deflector relative to the trailer can also be determined. The pressure sensor may comprise an air pressure probe with a plurality of inlets at a distal end thereof. A method of automatically adjusting the deflector is also described, with the intention of providing a more aerodynamic profile for the vehicle and thus improving fuel efficiency.
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