I spoke to Autometer about my American Platinum tach not working. This is the base gauge kit that FFR supplies.
When GM reflashed the ECM to delete the security, the rear O2 sensors, etc it sometimes interferes with the tach signal. He said you have a 50/50 chance it will work.
Mine doesn't work, I'll be looking into this today or tomorrow and posting more info.
In the meantime here is more info.
https://www.autometer.com/resources/...faq_view/id/77