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The most accurate reading would be from the ECU CANBUS (Flexray) bus, but you'd need a device to connect to it and also know the correct ID's to capture the right frames. Then there's the job of having to decode the hexadecimal data.
Another simpler solution would be to hookup an arduino/raspberry pi with a OBD interface and just use the OBD port. If you wiki OBD PID you'll see a list of generic 'request' commands for things like RPM, Oil Temp etc. I'm pretty sure the data is decoded too once you get it back.
The OBD method can't match the regular CANBUS for speed, but accessories like the P3 gauge use it for displaying basic parameters, so for visual purposes it should be adequate.
Can I ask what your trying to do with the RPM signal?
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