I'm guessing the accuracy problem is the senders. Although another possibility is oxidation on those nuts where the calibration resistor is mounted. If anyone has there gauge out of the car, can they please remove the resistor and measure it. My guess would be it's in the order of 500 to 1000 ohms.
If anyone has an old OEM sender, I will pay you $20 for it so I can run it through my testing setup.
Here is my test setup. I cover the aluminum plate with water so it should represent what the sensor sees in the car.