VSS and DM's SpeedCal

I am having a heck of a time trying to get my speedo correct. I recently bought a SpeedCal from DM to help fix it, but i still have problems. I have a couple of question that would help me to fix it.

Background:
I have 3.73 gears, a 21 tooth speedo gear (i know i don't need both, but i should be able to make it work), and the car is a 98 GT, stock tires.

Problem:
After lots of codes on the SpeedCal, the best that I can get is the Speedo to be correct in the 40-55 range, read to fast from 0-40 and too slow from 55-150.

Questions:
1. Can the VSS be bad (does that sound like a possibility)?
2. Would a physical move in the speedo needles position affect it (i.e. when the guage faces were changed to white ones)?
3. Any other possibilities?

Sorry for the long post, but I appreciate any help that can be given

thanks i will give them a call. BTW the VSS is the unit that has the plastic gear that goes into the transmission that picks up the output of the transmission. It then puts out a AC signal to the speedo. The SpeedCal goes between the VSS and the speedo and modifies the signal to correct an inacurate speedo.