History Lesson
I lately viewed both Grand Prix and Le Mans. I'd seen each at its theatrical release Stateside.
Grand Prix has the better...well, more developed...story line. The cars driven by the lead actors are Formula Three cars, with smaller chassis and engines, and arguably better suited for the necessary modifications for camera mounts and the like. You see cameos by Formula One drivers of the period.
Le Mans has the REAL cars with their REAL drivers during the 1970 FIA World Sports Car Championship. You see Group 6 Prototypes powered by 3-liter engines, Group 5 "homologated" Sports Cars powered by 5-liter engines, and Group 4 Grand Touring cars. Porsches 917K ("Kurz", short) and LH ("Lang Heck", long tail), 908 and 911 (perhaps a 914), Ferrari 512S, Lola T70s, Prototypes from Alfa Romeo and Matra-Simca, and I believe a smattering of Alpines and Chevrons, possibly an Abarth. For more pictures of Groups 5 and 6 cars and drivers from the era check the excellent links at http://www.imca-slotracing.com/.
Among the acting credits I spot one race driver, Jonathan Williams, but the "driven by" credits are bona fide. (Update: Austrian Erich Glavitza was an autocross, motocross and rally driver; Swiss driver Peter Huber perished during the 1983 British [moto] GP.) David Piper was no "stunt driver" but a competitive owner-driver of Lolas who also guested for Ferrari. There must be a reason Kurt Ahrens and Vic Elford are mentioned by name when the Porsche 908 pulls into the pits. (Update: That's possibly actual race footage from the 1970 LeMans event, for the car is the Porsche 917LH they drove; it retired fairly late due to engine failure.)
I've since learned no actual car was used in the crash sequences, but I've no doubt whatsoever those "re-created" race scenes were done at full throttle or close enough to count. McQueen and the others wouldn't have had it any other way. He himself had raced in FIA sports car events in preparation for filming, and a "camera car" entry by the film production company raced at and finished the 1970 Le Mans event (Williams co-drove).
Take away the lead Ferrari's flat tyre at the penultimate lap and you might as well see the end of the 1969 Le Mans event, quite literally an hour-long duel between two J.W. Automotive Engineering Ford GT40s and a "works" Porsche 908. The final result was Ford-Porsche (same lap)-Ford, and both Fords wore the same livery as the Porsches that finished 1-2 in the movie!