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I think OBD1 is not fast enough to display rapid variations, eg. RPM.
If you have a separate RPM-meter and a speedometer, I think they are digital (receiving digital signal 12V or 0V). In that case, use a serial resistor about 8KOhms and a 4.7V zener diode reversely to the ground after resistor. From this point a TTL signal can be directly lead into any uProcessor, like PIC. And this pic can measure short-time signals. Connect this PIC to the PC and transfer the data in certain period. For speed it is disturbing to display the value more frequently than 1 pro second. (Personal experience).
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Alopecia perniciosa
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