VSS goes to the PCM to inform it of vehicle speed. The PCM then sends a signal to the speedo on another wire based on the info it receives from the VSS.

I ran the wire directly to the gauge cluster, bypassing the buffer. Speedo is within 1 MPH in calibration. That's close enough for me.

The PCM must be flashed for the gear ratio and tire size/diameter for the calibration to be correct.