sunnycoastgreg: one thing to be careful of when using a “further voltage divider” to reduce the voltage from existing guage & sensor network. the additional voltage divider across the sensor adds some additional resistance in the lower leg of the “guage voltage divider”. so ideally you want to keep the overall resistance of the voltage divider high, however there are practical limits given the input impedance of the arduino analog IO . (i read somewhere that the voltage divider should not be higher than 10k (need to confirm this).
and hence with 10k across a typical tank guage sender unit, it does impact the reading a little. i did some measurements on my guages (VDO fuel and water ... with resistive sender units.. eg 0-180ohm) and the additinal voltage divider adds a few % error ... not enough to detect on the analog guage ... but if you plan to keep existing guage - you need to ensure that you calibrate AFTER adding the extra voltage divider
The tank sensor has about max 200 ohm (180 nominal) the others are in the same range. I have an overview from Faria where all this are written down. So if I use the dividers in KOhm area it should not be any problem or causing high error values. Thought to use 9.1K & 24K. This is a lot comparing to the 650 Ohm total the fuel sensor/gauge have. On the gauges, even they are high quality expensive once, there is nothing to adjust, unfortunately. BUT, If this really matter and an error is recognizable than a resistor in parallel to the gauge can compensate I guess.