When the development of the Ford Sierra was reaching its latter stages, management felt the need soften the buying public. Sierra’s styling, penned by Patrick Le Quement, was as avant garde as its predecessor, the Cortina’s, was conservative. So, the design team produced a more extreme version designed to continue the Ford Probe line of concepts and rolled it out at the Frankfurt motor show in September 1981 – a year before the production Sierra was due for launch.
The good reception this received encouraged Ford's management to go ahead with a production car with styling almost as challenging. This "aero" look influenced Fords worldwide.
The Probe III was a beautifully detailed concept. It featured overly aerodynamic features including Citroënesque enclosed rear wheels, smooth underbody detailing, flush glazing, and integrated door mirrors (which would appear on the production 1988 Probe couple no less). The car’s CD was 0.25, which was actually a world away from the production Sierra’s 0.34.
Source: Classic Cars Wiki