There are several errors in system. - reference voltage - noise from sensors to measuring system - measuring system own noice
So I am bit afraid that if you measure system voltage as own input and then sensor value, you may have noice between measurements and you do not actually get exact measure time difference. If required accuracy is not high, it should give reasonable values.
If it is poosible, I would rather stabilize sensor voltage first. If engine own gauges does not have any differential correction, also them would be stabilized.
For resistive measurement I do not know good way, if you do not know the range of measured sensor. That would require some auto-range system. It is far more easier to measure somehow the range and use suitable constant current source or voltage source+divider resistor. E.g. for tank sensor 30-240 ohm I simply use 3.3 V regulator and 330 resistor. Then I will get range appr. 1.4 - 0.3 V to input.