Sorry to dig that one out again but I'm still wondering...
Appearantly what's been recreated in LFS is a sound that the individual gearing pairs make when a specific gear is shifted. Thus at the same speed different gears result in different pitches. (Try on your own with the pit limiter and Shift+A)
What makes the awesome sound in the video of the first post seems to be the final drive though.
That would explain why the pitch of the sound is more or less directly linked to the speed of the car.
Maybe the LFS like sound is based on a real gear box whine? I'd like to see examples if anyone knows any video that shows this.
And again I invite any of the developers to state something or show examples.