I have a mess dropped in my lap. Local shop called asking for help with speedo calibration and I said sure no problem that's where it all began
1999 Camaro V8 6 speed customer swapped rear end from 4.11 to 2.73 and brought the car to them to get speedo working.
Tune details reveal this.
P01, ECM, GM Gen3 v8 4.3l V6

I drove the car as delivered with a phone GPS in comparison and at 60 MPH (GPS) the speedo was reading 73 MPH. (Gear/tire wizard showed 24.94 tire diameter and 3.42 gears ) stored a copy of the original configuration.
I went in using my MVP1 interface and input the tire diameter 275/40/17 @ 25.66 and gear ratio at 2.73 and wrote the file took the car out and it now was reading at 60 MPH (speedo) about 51 MPH (GPS).
reloaded the original calibration, took the numbers and ran it through a percent error calculator and attempted to scale the VSS pulse count for a -17.808 error and then I was running almost 30 MPH to fast.
I have been playing with this thing off and on over a week and cannot figure it out, I have never had this issue before but it's driving me crazy.
I am not sure what sort of mental block I am having here but I could use some ideas.
As a test I tried reloading the tire gear calculator with tire sizes from 24" to 27" and I see no correlation in the speedometer readings, I even changed the gear ratios from 2.73 to 3.08 and 3.73 but the speedo is either 30 MPH to fast or 10 MPH to slow. Is there something different about these cars vs all the other newer ECM/speedo combinations that I have set before successfully am unaware of? am I missing some fundamental steps or what?

Frustrated with myself not being able to find this solution.....