I knew someone would complain about the mark!
I used this term because looking at waveforms for different speeds I noticed the hi portion of the square wave seems to give good indication of the speed while the low portion sometimes gives longer periods. It must be a proximity sensor on gears giving the period proportional to speed, the fluctuation might have been when the driver was not exactly steady on the gas.
I think it makes no difference if I measure the period of the waveform but it will be much easier just to measure the hi portion of the square wave.
Mahmood