A Little Ohms Law or Some Help Here.

If it's a conventional gas gauge/sender setup, it should make a difference.

 

This is a common fault with the Mondial. I've never really looked into the cause. More likely this is a fault with the sending unit, and not the gauge.

 

Well the tank sender coil is simply resistance, so it is a function of the total voltage, but as noted the sender and the gauge are designed at 12V so over voltage would not make them read low, I would think the opposite.

I would say one or both are bad, but would start at the sender.
Used to trash them pretty often in my boat, as it was bow tank and took a pounding...

 

I believe the circuit is a wheatstone bridge so the voltage shouldn't make a difference.

 

I take that back.
The circuit is basically 2 voltage dividers. A variable side and a fixed side. The sender in the tank forms part of the variable side. The gauge shows the voltage difference between the 2 sides.

Say the fixed divider is feeding the gauge 6v and the tank side is in balance so it's also at 6v. The gauge reads zero because it's seeing the difference between the two sides. As the sender changes the resistance, the circuit goes out of balance and the gauge moves.

So yes, the 20v will make the gauge read an exaggerated reading.

 

Take it to gas station and see how many gallons it takes to fill. In the garage I started with an empty tank, and added exactly 5 gallons. Took photo of gas gauge. Then I knew on the gauge when to look for fuel!