John Frankenheimer revealed in an interview that none of the footage in the film is sped up. The stunt with the semi was done at the speed seen in the film, and they really did race down the highway in excess of 120mph. The latter required a mile of the highway to be lit, as the film was also really shot at night (not day-for-night).
The car seen in the final scene actually had been wrecked into a tree during filming. John Frankenheimer commended BMW for allowing the filmmakers to show the cars being heavily damaged in what is essentially a commercial. He also said that showing the BMWs actually being damaged, then continuing to race along at over 100mph illustrates just how remarkable these cars are.