raschemmel:
How much fuel in the tank when you measured 3.39V ?
Seemed to be about quarter to half tank. Don't know the exact amount unfortunetly.
terryking228:
When the fuel is empty, resistance is 3 ohms and when full it's 110 ohms.
Is this information from some car manual, or did you measure this??
My experience is that the resistances quoted in manuals is approximate at best.
Since you want to know the Voltage as the amount of fuel changes, I suggest you actually measure it. Maybe carefully hook up a 2-wire cable to:
(ground and gauge/level sensor wire; power on)
and connect your multimeter to it. Get a separate can of fuel, maybe 2 or 3 gallons, and drive the car carefully until it actually runs out of fuel, and get that reading. THAT is the one you care about the most. And many fuel gauges are known to be inaccurate near empty tank. Then use your spare fuel to add to the tank, restart the car and drive to a fuel station and slowly fill the tank, recording the voltage at 1/4, 1/2, 3/4 etc. THEN you can decide how to use Arduino to measure the range of voltages, and how you want to display "Liters Left before the car stops".
Do not do this on the AutoBahn.
I love Engineering, but sometimes the Empirical approach gets you what you need.
Thanks for your input!
I took the sensor out and measured its resistance in various states. Lowest was 4 ohms (which was when the tank would be full) and the highest was 111 ohms (which was when the tank would be empty). So it comes pretty close to the range claimed in cars manual. 
The programming part is not the issue. Issue is with the circuit part, as I don't know how to design such a circuit which reads the voltage but does not interfere with the orginal gauge circuit...
When this gets sorted out, there's still a problem with variable voltage. When car is not running, the voltage of the battery is about 12 volts. When the car is running, the voltage is about 14.4 volts. So the voltage measurement gets a little difficult. I must also program the Arduino to sense if the circuit is running at 14.4 volts or 12 volts.