So here in detail how you get the fuel gauge get to work:

Pin 1 and 2 of the dashboard connector are used to measure a resistance between these pins and GND.

Is0-Mick had a very nice idea. With it you can also show the values of the in-game fuel level. The plugin from Silas Parker does also deliver values of the actual fuel level. Read those with your Arduino and replace the potentiometer in the upper circuit with some resistors (I used besides the two 68 Ohm resistors five 38 Ohm resistors). Between those resistors add a cable to a pin of the Arduino. In this way you can shortcut pins with the Arduino and change the resistor value.

Thanks also to @Pranas who reminded me to add this information.