The RS8 wasn't a concept
It was built using some parts from Kugel and other parts made for Ford, rear suspension and part of the steering was different to the Kugel kit.
It was built to showcase the new Cammer V8 which was developed for aftermarket use, not for production cars.

The RS8 was never intended as a road legal car to be sold by Ford. I believe 2 were built and they were only ever demonstrated on offroad circuits.

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