oh it's totally possible to make such a thing
i would use something like a raspberry pi and some off the shelf hardware to make it easy
the raspberry pi would run linux or whatever and would simplify things since you could use a store bought ELM interface for the J1850 VPW side and then you could use any old FDTI TTL chip of the 8192 side
of course it could do much more than just your tach
a simple program running in a loop could become bus master and transmit the ALDL message of your choice while also querying the OBD-II bus at the fastest rate possible
i've done stuff like this lots but to get it done you have to have some basic coding skills
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