Hi SIFLIGINIUS.
I don't think your question is particularly relevant for this forum, but I will attempt an answer.
If your gauge readout is analogue, with a moving needle, then you don't need to "convert the voltage" at all. Just print a new scale for the gauge and stick it on top of the existing one.
If your gauge readout is digital, with an LCD display, then first of all I would double-check if there is a "voltage at the instrument". It may have a digital interface, in which case you need something like a microcontroller.
If your gauge readout is digital, with a voltage input, then yes, you could use an op-amp circuit to perform a conversion, but would it really be any use?
Chances are the gauge isn't doing a straight-forward analog-to-digital conversion. There's probably a microcontroller in there too, with a look-up table or scaling factor involved. You would have to take the instrument out of the car, connect it to a programmable voltage source, and play around with it to be sure.
e.g. See the Difference Amplifier here:
If your input range is 0 to 5V, then choose V2 = 5V, and R1 = R2 = R3 = R4 = 10k.
Then when V1 = 0V, Vout = 5V, and when V1 = 5V, Vout = 0V.
That gives you the "reciprocal" function you're looking for, but that's only part of the solution.