- Will tapping the sending unit's wiring for input into the Arduino alter their accuracy at the in car gauges (or Arduino)?
It shouldn't if your car gauges are low impedance. In any case, you can/should use OpAmps as voltage followers or buffers to tap the signals and connect them to Arduino. This will avoid loading the instruments too much and provide the right signals to Arduino. You may even need to set some amplification to the Op Amps and invert signals or add DC levels depending on what the gauges get to work with. It's not difficult to do that. Check the signals you will be tapping and post here your measurements so we can help you better.
You can check these configurations to get familiar with the subject.
-Summing Amplifier
-Voltage Follower.
-Non Inverting Amplifier
-Inverting Amplifier.
- Can anybody recommend a readily available application (Windows or Linux), that will allow me to display the received data on the laptop - preferably as gauges?
-Labview maybe?
-You can always make your own code; but that's going to be a bit harder.
You have come up with a nice project idea.
Good luck.