I have something in the works, But I need to know how to make a fuel sending unit that reads 40 ohm to 250 ohm to work with the fiero gauge. Like maybe a resister in the wiring. Thanks

electrical

The Fiero sender is 0-90 ohms, if the one you have now goes from 40-250 ohms, it's already out of spec and adding a resistor will only make it further out of spec. The resistance is a function of the windings, so unless the one you have has a resistor soldered into the circuit that you can remove, it wont work.

You could do it with an Arduino.