Speedo error conversion related

[js1973]

I recently changed a wh statesman from the sc v6 back to a standard v6.
The issue i am having is speedo related.It seems to work ok up until 105 k's an hour
then has a fit under load up hills the speedo drops back to 80 k's and gps says i am doing 120.
Plays havoc with the cruise control as well.
Any ideas would this be wiring related or tune related i am using the original 2000 model supercharged gearbox etc with the vt powerchip tune.
And the v6 non supercharged wiring harness.

[VL400]

Welcome

Have you checked the connector for the speed sensor, even if you just disconnect/reconnect it. Might just be a bad connection. If you have a data interface log what the PCM see's the speed as. It passes this (and other things) on to the dash in a data frame. I'd be surprised if its the powerchip, but if you have a programmer its not hard to load a standard bin to test.

[js1973]

Yes i'll try the standard tune and try and log though it seems strange though that it is worse up a big hill and only happens at over 100 k's.
The steeper the hill the further out the speedo gets.
Is their any major differences between the gearboxes wiring etc other then the torque converter.
My original tune for the sc engine was a darc 4389
The one that came with the engine is a cxwl 1938