stupid question.... how do you know its 20 degrees off ?
Here is an easy way to check.
If you have a thermostat in the water neck and you know what it is (say 180), with the car cold, fill the radiator to the top and leave the cap off. Start the car and have someone tell you the moment the temp gauge reaches 180. At EXACTLY 180 the thermostat opens, and fluid starts flowing. The level will visually drop a little and you'll notice bubbles forming from the current of fluid. The thermostat doesnt lie. This will tell you exactly how off the gauge is. If the gauge is inaccurate you should be able to use any sender as long as the ohms are correct. (assuming it is an electrical gauge)