1966 saw the advent of a completely new set of regulations from the CSI (Commission Sportive Internationale - the FIA’s regulations body) – the FIA Appendix J, redefining the categories of motorsport in a numerical list. GT cars were now Group 3 and Prototypes were now Group 6. Two new classes for Sports Cars were Group 4 and Group 5 for ‘Special Sports Cars’ (Group 1 and 2 covered Touring Cars, Group 7 led to the Can-Am series, with Group 8 and 9 for single-seaters).
As Group 7 were ineligible for FIA events, the Automobile Club de l'Ouest (ACO) opened its entry list to Group 3, 4 and 6. The FIA mandated minimum annual production runs of 500 cars for Group 3 (up from 100 previously) and 50 for Group 4, which also had a maximum engine capacity of 5000cc. There were no engine limits on the GTs or Prototypes. As before, the Groups were split up in classes based on engine size, there was a sliding scale of a minimum weight based on the increasing engine size (from 450 to 1000 kg for 500 to 7000cc) as was fuel-tank capacity (60 to 160 litres).
Along with the new Appendix J, after four years of focus on GT racing the FIA announced the International Manufacturer's Championship, for Group 6 Prototypes (2L / >2L), and the International Sports Car Championship for Group 4 (1.3L / 2L / 5L).