04 silverado 4.8 T56 magnum speedometer calibration problems

Have a 04 chevy Silverado 4.8. i recently swaped the NV3500 Trans for A Brand new TUET11012 tremec magnum t56 6 speed trans. i am Having trouble calibrating the speedometer so it can read right. i have tryed several things,
Change the Vss Rev-trans to 12 (since those trans come with a ford style Vss sensor)
Changed it to 17(Believing it had a gm one)did the caculations usin the gear and tire wizard.
And tried doing the calculations manually by Measuring the distance traveled in one rev of tire And inputing those values.
Can anyone help? Wha Am I missing!?

I?m having a similar issue with my Toyota pickup swap. Running a t56 magnum as well and the readout on The scanner is way off.

This isn?t that big of a deal for me since I?m running a GPS based autometer speedo. But I?d like to get it right, since the tune pcm does have some tables that are speed dependent.