How can I make 20-200 ohm variable resistor look like a 20-240 ohm resistor?
I have a fuel sender in my kit car that goes from 20R to earth (full) to 200R (empty). Unfortunately the gauge (constant current source, measuring resistance to earth?) expects empty to be 240 ohms, so displays '1/4 full' when empty. 


Status   Spec.   Actual   Reading  Voltage
------------------------------------------
Full       20 Ω    20 Ω   100%      160 mV
Empty     240 Ω   200 Ω    25%     1600 mV
                  Required voltage 1920 mV








simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. (a) What you've got. (b) The voltage reading can be boosted to the correct level by feeding an extra 1.6 mA through the sensor. (c) A simple current source based on 12 V supply and 1920 mV for empty.


  What is the simplest analogue way to make 20-200R look like 20-240R?


R4 looks simple to me.

I suspect that the 8 mA source is constant current so that the gauge doesn't fluctuate with revving of the engine (other than that gradual downward trend as you burn up the earth's carbon fuel reserves). To avoid R4 introducing variation you might want to feed it from a stable voltage source - as high as you reasonably can - and recalculate for your new voltage.

At full the extra 1.6 mA will increase the voltage through the 20 &Omega; by 32 mV. On a span of (1920 - 160 =) 1760 mV this represents an error of 36 / 1760 = 2% so with a full tank it will read about 98%. This should be acceptable.