Resistance Conversion/Scaling Circuit
Generally speaking when information is transferred in an analog format, it is by varying a voltage (as opposed to some other electrical quantity like current or resistance). This is because the instrumentation for conditioning and digitizing voltages is simple and easy to use. Somewhere in your truck there are electronics that are measuring the resistance of your variable resistor sending unit by converting it to a voltage; it may be simpler for you modify that circuit to work with your nonstandard unit than to try and make your own middleman variable resistor. 

That being said, if you absolutely must convert your 40-250 ohm variable resistor to a 0-90 ohm, the way to do it is by constructing a voltage controlled resistor (VCR). This can be a complicated circuit to build; a good place to start is with this answer: Floating voltage-controlled resistor with LM13700: How does it work?