I've decided to map the gauges as such:
Gas Fuel Gauge: 0-4/4 = Battery State of Charge - driven by OrionBMS SOC output (0V-5V)
Oil Pressure Gauge 0-5 Bar = Pack Amperage 0-500Amps - driven by OrionBMS output (2.5V-5V)
Temperature 100F-250F = Motor Temperature 100F-250F - driven by motor temperature thermal diode
I had to craft a few custom circuits to convert the outputs of the OrionBMS and motor's thermal diode to the input requirements of the ECU. Here are some sketches and a picture of the circuit that drives the Oil pressure and coolant temperature gauge.
So far I've built up the Amperage and Temperature circuits. I've debugged and tuned the Amperage circuit, however, the Temperature circuit is giving me trouble. I'm going to try a single ended op-amp in place of the dual-supply part I have in there now. Both the circuits use one op-amp to multiply the input by a number, then a second op-amp to subtract a fixed offset. I've only done limited testing in the car so far. I need to finish debugging the Temperature circuit before I hook it up to the motor's embedded thermal diode and the car's ECU.