The first J-car was completed in March, 1966 and set the fastest time at the LeMans trials that year. The tub weighed only 86 lb, and the entire car weighed only 2660 lb, 300 lb less than the Mk II. It was decided to run the MkIIs due to their proven reliability, however, and little or no development was done on the J-car for the rest of the season. Following LeMans, the development program for the J-car was resumed, and a second car was built. During a test session at Riverside International Raceway in August 1966, with Ken Miles driving, the car suddenly went out of control at the end of Riverside's high-speed, 1-mile-long back straight. The honeycomb chassis did not live up to its design goal, shattering upon impact, bursting into flames and killing Miles. It was decided that the unique, flat-topped "bread van" aerodynamics of the car, lacking any sort of spoiler, were implicated in generating excess lift, and a more conventional but significantly more aerodynamic body was designed for the Mk IV.