I've heard that the sound was recorded from a GT40, which is a six speed, which would explain all the shifting.

If you watch the tail pipes closely near the end of the chase (after he spins out in the gravel) you can see puffs of smoke when he actually shifts. Some match the sound but others don't. Also, earlier in the chase when he turns left and goes up hill, the sound doesn't change, but the video jumps back a little.

Or maybe I just watch it too much.

 

A 6 speed in 1968?......I'm pretty certain they didn't have any 6spd. back then.

 

You could have me there, I'm not sure what they came with. I assumed they had a five or six speed, since they were raced in Le Mans and such.

 

Actually according to an article on the anniversary of the film he did the driving until he got a little too out of hand during one stunt and the director pulled him in favor of a stunt driver as it would have been hard to finish the film if McQueen was dead.