MOELLER
Electric Fuel Gauge Sending Units
These sending units fit the industry-standard bolt-hole pattern and 240-33 Ohm gauges. Internal measurement components will not rust or corrode.
So, the sender is expecting a resistive gauge. If you intend on using with an existing gauge, you will need to "pull-off" from the sender-meter connection and use a high-value resistor divider to scale the voltage for the Arduino Ax input. I would probably use a 10K linear potentiometer with the wiper going to A0.
If however you are going to use without a fuel gauge, you need to supply a low value resistor to take the place of the meter. Maybe 100 Ohm. But, a 200 Ohm rheostat could be used to fine tune things.
Ray