My tank gauge is both reliable and can be calibrated. It's not an electrode sticking into the tank. It's not a sonar stuck on the side of the tank, no, It's a pressure gauge that's at the bottom of the tank.
The more pressure, the more full the tank. When they installed it at the Factory, they calibrated it for zero full and 100% full. Now it reads accurately in 1% increments.
I don't understand why all manufacturers don't use a pressure gauge.