According to the article that I read in Muscle Car Review back in the eighties, the director and the guy who prepared the cars said that there were two Mustangs, and two Chargers. Steve McQueen and the director decided that there were too many Fords,and to make the film more realistic they decided that the villain vehicle needed to be something different. The director and the mechanic went around to a bunch of local car dealerships looking for the right car to be the villain car. They bought two 68 Dodge Charger R/Ts one blue,and one yellow. The blue car was used as the stunt car,and was an automatic transmission equipped car, and the other car was yellow, and was a 4 speed car. They were both painted black to appear more sinister looking. They took the cars to a local racetrack the day before filming and they said the stock Chargers ran rings around the Mustangs, and they tried to handicap the Chargers by running smaller tires. They addad a cam,carb and intake upgrade to the Mustangs and the stock Chargers were said to just pull away from the Mustangs like you wouldn't believe,said the mechanic! They did some gussets on the suspension systems of the two cars used in the hill jump scenes, but other than a tweak and tune sessions the Chargers were unmodified.