Motor Trim Explained:
Motor trim allows the entire speed range of a decoder to be shifted up (faster) or down (slower) using CV 66 and CV 95. This makes speed matching locomotives for consisting as well as adjusting for those notoriously fast and slow locomotives a breeze. If your locomotive performs slightly differently in forward than reverse you can adjust the trim in both directions independently to even out the speed curve regardless of direction. Motor trim can be used on top of User Loadable Speed tables and 3 point Speed Curves to adjust the entire range up or down. This way if you have the perfect speed curve but your locomotive is still slower or faster than you would like you can shift your custom curve up or down without adjusting each individual value.
Motor Trim Values:
As outlined in the diagram above, a value above 128 will increase the overall speed (the higher the number the faster the locomotive will go) and values below 128 will slow the locomotive (the lower the value the slower the locomotive will travel. The resolution (precision of the adjustment) of the values for motor trim is 4. This means that for every 4 you add or subtract from the trim value the speed will be adjusted by one degree.