Lindsay;
I've been dealing with this same problem on my '49 GMC that has a 261 Chevy engine in it. My gauge is original electric and a range of 100-220 degrees and the sender on the 261 is in the thermostat housing, not on the head. From my experience, you can't add a resistor inline to the gauge to make a difference in offsetting the temp reading (I tried this with all kinds of resistance ranges). This is simply because the sender is a NTC type resistor which stands for negative temperature coefficient. Basically it means that as the temperature rises, the resistance falls and vice versa. The early GMC's used an electric gauge where the Chevy's still had the mechanical one. The problem is matching a sender with the proper NTC resistor to the original gauge range. The old GMC senders are long gone and the newer ones do not match the range. I am going to experiment on a gauge that I just bought where there are two adjusting nuts which are factory sealed on the back of the gauge. I believe that by moving these nuts, it will change the resistance of the gauge and then I can match it to sny sender. The problem will be in getting the sender in water at 100 degrees and keeping it there while adjusting the gauge and then raising the temperature to 220 degrees and keeping it there while I adjust the gauge. Think that it may work but won't know until I try.

Country Roads, West (By God) Virginia
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