Someone please correct me if I am wrong.

The speedometer uses a sensor after the differential so the only thing that can mess up your speedometer is tire diameter.

 

Nope, speed sensor (for the speedometer) is on the tailshaft of the transmission.

 

I did a search and found a few posts stating the GS uses the ABS sensor which is located at the wheel. When you look at the "I just installed my 3.76 differential" threads I didn't find anyone making changes for their speedometer.

 

On my car, 2005 GS3, if I disconnect the speed sensor on the trans the speedo stops working. When going to a 3.73for a 3.26 rear end gear the speedo will read ~15% higher than actual (assuming stock wheel diameter is retained).

 

I think the cable comes along the drivershaft towards the right rear wheel spindle. It's reading the rotation of the rear wheel.

 

That's interesting. I'll have to disconnect the rear wheel sensors and see what happens. Maybe the trans sensor is just some type of backup sensor.