Check out the video at 3:07
1. Try to match the frequency of the observed sound which I estimate is around 350 Hz.
2. Figure out the speed of the car away from camera. It sounds like the top of 3rd gear which should be around 90-100 mph depending on gearing.
3. Correct for Doppler effect. Easy if you can Google.
http://www.wolframalpha.com/input/?i...400Hz%2C+90mph
So again, it sounds pretty close to 8000 rpm. It's way better than a guess.