What you are explaining are calibration curves. For every German car over the last 20 years, the temp is transmitted to the CAN. The instrument cluster reads that info (i.e. wheel speed, engine speed, fuel level, check engine light, etc.). For each gauge, there's a calibration curve. It's an X-Y plot. So X is the number the ECU transmits and Y is the displayed number in the cluster. Technically, it should be linear but it is not. It might be linear to a point and then the curve becomes parallel to the X axis for a period of time (i.e. if the gauge reads 190 that could be everything from 170 – 220) and then will go up again.
For example, on an e46 BMW, the water temp reads just slightly over half. When we stick in an AIM dash and read the info directly from the CAN, the temp is anywhere from 170-225. It literally will go from that normal area to the red at 230. Originally, AIM made a mistake and the water temp always read 20 degrees too hot from actual. So, the calibration curve had to be changed to get it to read correctly. Even today, there's a company that you can send your e46 instrument cluster to and they will reprogram it to make the gauge read correctly.
As for your VW, best thing to do is to read the temps off of the CAN and then read what the cluster indicates. Then plot those numbers. At that point, you'll see what VW did and how far they are off from the real temp. You can either reporgram the cluster to read correctly or buy and AIM dash, or take the X and Y numbers, print them out and tape it to the dash or get one of those apps and a bluetooth OBDII reader with CAN capability or just plug in your VCDS link it to your phone. Maybe there's a way to get it to display the channels from the CAN.
What you need to do is log those X and Y numbers and post them. Then we will see how far off the cluster is to reality.
