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The cars : Ford Sierra development story
https://www.aronline.co.uk/cars/ford/sierra/the-cars-ford-sierra-development-history/

The Sierra had been developed without prior knowledge of the C3-generation Audi 100’s highly aerodynamic shape, which made aerodynamics de-rigueur. Of then-current production models, the Sierra’s drag coefficient of 0.34 had only been beaten by the Porsche 924 and Citroen GS. And as for its middle market rivals, no-one had come close to that figure.

Referred to as an ‘aeroback’ design by its creators on account of its bustle at the rear, the Sierra sported a similar body shape to the recently launched Escort Mk3 (known internally as Erika). The later XR4i improved on the standard car’s Cd figure by incorporating sculptured side panels to control air resistance across the wheel faces, as well as its iconic bi-plane rear spoiler. The result was a very impressive Cd of 0.32.

................This revolution in mainstream car styling was impressive enough for the Sierra to be displayed in England’s Victoria and Albert museum during 1982.