Well, I just purchased a NOS AC Service parts GMC gauge part number 1511960 from a supplier in Canada. 100-220 degree range. I'm looking at the back of it now in my hand and there are two areas where a small hex bolt head is covered with yellow paint. It looks like once the hex head is loosened, it allows you to slide the interior wire wound resistor coils approximately 1/32" either way from center. I believe that by doing this, you can adjust the range of the gauge to match the range of your sender. However, there are no markings to tell where you are. When I look at the wire wound resistor coil at the 220 degree end of the gauge, the hex screw is all the way to the left or to the outside rear of the gauge face. At the 100 degree end of the gauge, the hex head is about 1/64" away from the far right side of the rear of the gauge face. I would surmise that at the factory this is the way AC Service Parts "zeroed" or calibrated their gauges. In order to make your gauge work properly with the sender in your truck, I would guess that you would have to remove the gauge and sender, set up a test stand like I mentioned above, put the sender in water and heat the water to 100 degrees, zero the gauge, heat the water to 180 degrees, adjust the gauge and heat the water to 220 degrees and readjust the gauge. Lots of work but it looks like it can be done. A winter project if I want to try it for sure. I'm now trying to find an original GMC electrical sender from '49 to '52 or so. One contact told me that the same sender for GMC trucks was for either 6v or 12v gauges. Will let you know if and when I can find one.
