I believe the MTH standard is 24 stripes on the flywheel. However, I haven't actually counted them on every model. 
Ted S posted:I know from my own experience that the flywheel diameter varies by application, even among diesels. @gunrunnerjohn, is it true that all locos have 24 stripes from the factory? I know that you've researched the characteristics of the sensor... From your experience, as the motor RPMs increase, does the tach signal increase linearly, logarithmically, exponentially, or ??
The speed difference is simply the combination of gear ratio, driver diameter, and flywheel stripes. It's a linear computation, as it's a digital relationship. The diameter of the flywheel has no bearing on the speed computation, it's the number of stripes on the flywheel.
Ted S posted:Bottom line, if I were to swap the file into a known loco, the error between the observed speed and the commanded speed could be due to a different gear ratio. But it could also be due to a different flywheel diameter, stripe width, number of stripes, etc. Why don't they just put the gear ratio on the darn box!?
The PS/2 speed is determined by the sound file, the gear ratio, the driver diameter, and the number of flywheel stripes.
