The nickname "J-Car" derived from the car's construction to meet the new Appendix J regulations introduced by the FIA in 1966. The project represented a clean-sheet departure from the steel spaceframe construction of the Mk I and Mk II. At its core was a chassis tub assembled from aluminium honeycomb panels bonded together — a structural approach borrowed from aerospace construction, developed in partnership with the Brunswick Aircraft Corporation. The resulting tub weighed only 86 lb (39 kg), and the complete car came in at approximately 2,660 lb (1,207 kg), around 300 lb (136 kg) lighter than the Mk II.
The J-Car was produced through Ford's performance subsidiary Kar Kraft under Ed Hull, as part of a broader shift to bring GT40 development in-house in the United States. Ford Advanced Vehicles in England, which had managed the original Mk I programme, was sold to John Wyer as part of this reorganisation. In this sense the J-Car was the most American GT40 variant attempted, preceding the fully domestic Mk IV.
The first J-Car was completed in March 1966 and set the fastest time at the Le Mans trials that year, suggesting the platform's raw potential. Despite this, Ford's racing management chose to run the proven Mk IIs at the 1966 Le Mans, with the J-Car requiring further development.
A second prototype was built and testing resumed. Ken Miles — who had already won the 1965 Daytona 2000 km and the 1966 12 Hours of Sebring for Ford, and had been controversially denied outright victory at Le Mans 1966 by Ford's orchestrated photo finish — was the primary test driver for the programme. During a test session at Riverside International Raceway on 17 August 1966, the car suddenly and inexplicably went out of control at the end of its high-speed, approximately one-mile-long back straight. The aluminium honeycomb chassis shattered on impact and the car caught fire, killing Miles.
The accident investigation concluded that the J-Car's distinctive flat-topped aerodynamic body — which experimented with Kammback aerodynamic theories and included no meaningful spoiler — generated significant aerodynamic lift at the speeds reached on the straight. The chassis structure, while strong in normal conditions, also had no roll cage, and the combination of aerodynamic failure and insufficient occupant protection proved fatal.
Following the accident, Ford's engineers undertook a thorough redesign. The distinctive "bread van" body was discarded and replaced with a more conventional but aerodynamically superior shape, carefully developed to eliminate the lift problem. A NASCAR-style steel tube roll cage was incorporated into the chassis, substantially adding weight but also significantly improving driver safety — a decision vindicated at the 1967 Le Mans when Mario Andretti crashed violently at the Esses and escaped with minor injuries.
The revised car was designated the Ford GT40 Mk IV. It retained the J-Car's aluminium honeycomb chassis as its structural basis, along with the 427 cubic inch (7.0-litre) Ford V8 engine, gearbox, and some suspension components from the Mk II. The Mk IV ran in only two races, winning both: the 1967 12 Hours of Sebring with Mario Andretti and Bruce McLaren, and the 1967 24 Hours of Le Mans with Dan Gurney and A.J. Foyt.
A total of nine cars were constructed with J-Car specification chassis, six designated as Mk IVs and one rebuilt as the Ford G7A for Can-Am racing.
The J-Car occupies a painful place in Ford motorsport history. Its loss of Ken Miles — one of the most capable and experienced endurance racers in the programme — came less than two months after Miles had been passed over for the Le Mans win he arguably deserved. The car itself demonstrated both the promise and the danger of aerodynamic experimentation and novel construction methods in an era when wind tunnel testing was only beginning to be applied systematically to racing car development. The honeycomb chassis technology that the J-Car pioneered became standard practice in high-performance motorsport in subsequent decades, seen most prominently in Formula One monocoques from the 1980s onwards.