Especially after that very short closeup scene where the Mustang's front end went into a ditch and broke it's front strut to the point front tire hangs down.

Miraculously, however, McQueen throws it in reverse, and is speeding along again.

 

They used two Mustangs in the movie (and two Chargers)....one was used for close-ups and one for stunts.

Eventhough the stunt Mustang had added structural reinforcements to stay in one piece, after all of those SF street jumps...the car had so much structural damage, that it was junked.