SSoon after Ferguson's death, his son-in-law Tony Sheldon took over the chairmanship of Harry Ferguson Research and changed the company policy to one of developing 4WD systems that could be adopted by car makers for their own models. Jensen Motors took up the idea, stretching the Interceptor by 5 in (130 mm) to create the FF (for "Ferguson Formula", Ferguson's term for 4WD). It appeared in 1966. The high cost of its hand-built 4WD system kept it from being a commercial success. The company also converted a number of Ford Mustangs to 4WD with the aim of getting Ford in the USA to make them as a production model. In 1969 the company converted a fleet of   Ford Zephyr MkIV police vehicles for assessment by the UK government, with interests in possible military use.