wiring diagram and figure out how the network is laid out.
You could just as easily compute some of those things from the ECM. Range is the % fuel left in tank versus average fuel economy. Instantaneous consumption is based on the injector flow rate. The display isn't doing anything too crazy. It's more a matter of knowing how to read the data out of the ECM if you're going to write custom software.
- High speed (500kb) two-wire CAN on pins 6 & 14. Typically used for Generic OBD2. Of course Mongoose GM can do this, but they don't sell Opel in the USA so I've never tried one.
- Medium speed (up to 250kb) two-wire CAN. Different name but only the baudrate and pins (typically 3 & 11) are different. Since Mongoose GM only does CAN on 6 & 14 you'd need to reroute the pins, connect at a lower baudrate, and start hacking.
- Low speed singlewire CAN. It's still called CAN but electrically it's totally different. Mongoose GM doesn't do that one (yet).
Hope that answers your question.