Denny, you are assuming these were calibrated from the factory. At best +/-2%. It is important to match sensor resistance with gauge impedance.
You can offset the gauge reading with a resistor at the back of the gauge. Depending on the resistance swing of the sender, you will either have to go parallel or in series with the sender. using a volt-ohm meter, if the sender resistance goes up when it gets warmer, add a small resistor (10 ohms, 1/2 watt) to the wire to increase the gauge reading. If the resistance of the sender goes down when it gets warmer, then you will have to parallel a small resistor to ground (about 1000 ohms, 1/2 watt). You may have to pick up an assortment from radio shack.
