Ok... I downloaded several sound files and looked at bytes 0x08 and 0x09. I think byte 0x08 might be a representation of motor revolutions per distance traveled. It seems to be "54" (decimal 84) for all diesels. Every MTH diesel I know of has a gear ratio of 10.5:1, and wheels that are about 0.875 over the traction tires.
So what is byte 0x09? Well, in order to determine motor RPM and train speed, the tach sensor has to count stripes, or at least interpolate the motor RPM by measuring transitions between light and dark marks on the flywheel. The relationship between number of transitions and train speed is a function of the RPM, but also of the flywheel diameter, the number of stripes, and the width of those stripes.
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 ??
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!? 
