Do you know the pot's maximum and minimum values?
Anyway, as I see it, you need to get in hand a voltage supply that can provide more than 5 volts AND less than zeero (to use opamps properly). The best situation is that if that voltage supply is electrically isolated (right term to use?) from your ECU, exept for the voltage (0-5V) terminals for measurement.
Otherwise, you need to make a circuit that can pump up the voltage to >5V and <0V. It's not very difficult to achieve. You need an oscillator with low impedance output and a couple of clamping circuits to get the higher/lower voltage out.
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I think this is something like what im trying to achieve except the pot on this is 5k (r3) i think. What would i need to do to use the 118 ohm pot.
Is there a voltage divide or something i can do here ?
You might get an opamp that have full range inputs combined with rail to rail output. I don't have that part's name, but unless you can find it, you need to have a wider voltage span than 0-5V to supply the opamp.
And since this is a fuel gauge, I guess that having a non-linear measurement is a bad thing (gauge will fall much faster when the tank volume getting closer to zeero).
And of course, having the exact max and min values for the pot is critical to achieva e correct measurement and further calculations of resistor grid around the opamp.
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My thaughts have been for a wacation, apparently. If your spare input is fairly accurate, then you can easilly construct an almost-linear gauge by meassuring voltage near 5V (or near 0V).
What is the resolution of the spare input?