How to count bike wheel Revolution by 3-axis acceleration
If you want to just count revolutions, you do not need to analyze sine wave.
Just count peaks of that signal.

UPDATE:

Of course, with some hysteresis. I take it as natural thing.

I would do max(v(n), v(n-1)) for every sample, until v(n) is like 10% below that current max value, a then start over again for min value.

This is the way I would count high peaks and low peaks. Then it is easy to calculate RPM.