I've been getting quite a few questions from people that are having issues with getting the transmission to shift at the correct time.
Ford PCMs calculate a value called N/V (N over V).
N = Engine Speed
V = Vehicle Speed
In the screenshot above is the starting value for N/V. This is the N/V the stock shift schedule was calibrated for. If there were axle ratio changes, or tire size changes, this value is wrong.
The PCM can learn the N/V somewhat, but there are clips for how far it can go. If the PCM ends up clipping the N/V value, you will run into issues getting the transmission to shift on time.
The Base N/V value is divided by the N/V it comes up with (clipped or not) and the resulting value is a correction factor. This correction factor is used to adjust the vehicle speed coming from the VSS.
Bottom line is the Base N/V needs to be pretty close to accurate.
I've found the easiest way to determine what value to use is to run the car on a dyno, in the 1:1 gear ratio with the converter locked, and look at the calculated gear ratio. (See Below).