incorrect coolant gauge reading
 

My coolant gauge does not appear to be reading properly on my 1985.5 NA 944. Using the instructions at Clarks I hooked up a variable resistor and can position the coolant gauge needle with the following resistances:

3.4ohm, ~100degC
38ohm, ~80degC
50ohm, ~50degC

And at room temp (car had been sitting over 8hrs) the coolant temp sender has a resistance of about 560ohms. When the car is heated up the coolant gauge reads at about 50degC. I didn't think to measure the resistance, but with the gauge being at about 50degC I'm guessing the resistance was about 50ohms which puts the temp between 80 and 100degC using Clarks chart (fans were on at that point).

So I'm thinking the issue is not with the sending unit but with the gauge.

But I'm curious if anyone has seen this before or if anyone has any other ideas? The previous owner did replace the interior of the car (possibly from 924), not sure about the dash, but could the cluster be a mismatch? Could a bad ground cause these symptoms? can I just add a ~33ohm resistor in there?

Thx!