Real locomotives run at various top speeds depending on their service and the gearing or configuration necessary to work properly in their environment.
To be practical in the model world and to be able to consist the locos, set a standard top speed (for your railroad) and adjust each decoder so that the locomotives match the standard.
1. First, pick a top speed remembering that distance is usually compressed so a slower speed means more time between stations.
2. Apply a basic set of values to the decoder such as:
a. Use the long address (normally the loco number)
b. Turn off DC (analog) control.
c. Set it to use 28 speed steps. This is important as manufacturers implement the 128 speed step function differently and don’t always match each other.
d. Insure CV3 and CV4 are 0 (zero) for now. You can go back and add momentum later.
3. Before you attempt to set the speed controls, run the loco for a few minutes to warm it up and insure the lubricants are distributed to the bearings.
4. Pick a distance (I use 24”) for a speed trap and mark it on the layout or test track. I use a spread sheet table I created that matches scale speeds to the seconds to travel the selected distance. Click the link or find it under “tools” on this web site.
5. Next, adjust CV2 until the loco just starts to move. There are methods to adjust the BEMF to help with this but that is another story. Use “Programing on the Main” for these CV settings.
6. Assuming the decoder can use CV5 and CV6 for the speed chart, run the loco through the speed trap using speed step 28 and measure the time. A stopwatch is great for this. Adjust CV5 until the time through the trap matches your desired top speed.
7. Now, set CV6 to one half the value in CV5 and check the time running at speed step 14. This should be one half the top speed but may require small adjustments of CV6 to achieve this. If your value for CV2 is higher than about 2, you can add the values in CV2 and CV5 together and divide the sum by 2 to find a starting value for CV6.
8. Now I would apply acceleration momentum to CV3 keeping the basic timing matched between locos.
9. Do the same with CV4 for deceleration adjusting for braking if you use that.