Because I'm currently bored at work, I thought I'd work out my RPMs from a video I put up at the start of this thread. I worked it up using audacity in the same way an audio tachometer would do.
The top is a clip of the sound from the video and the second one is the time-based FFT.
For those unfamiliar, it's a plot of frequencies present in the sound clip (AKA the harmonics of my engine sound).
The next part is the average frequency spectrum plotted for the sound clip.
This gives the average information about the whole 5 seconds that the clip contains (making it much more accurate).
The engine harmonics show that it's basically 12Hz, maybe more like 12.5, but no real biggie.
So 12Hz*60sec = 720 explosions per minute.
However, I think that 4 stroke engines do 2 rotations per explosion, so 1440 RPMs.
I think that is pretty high right? I prob am wasting fuel at red lights....