Is 12,000 rpm's to high

If you can afford it I would go with an aftermarket tach. I have an autometer and the factory hooked up and the factory tach is close until about 3500-4000 rpms. After that the error grows exponentially. But if you want both or just the factory tach do as the others have said.

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88GT stock shortblock, ported GT-40 iron heads, ported Edlebrock Performer intake, 24# inj, Pro-M 75mm Bullet MAM, 3.73, hurst short throw, F-303, electric fan, alum rad, FMS HD clutch, R134a ac conversion, 70mm TB , and some other stuff I probably forgot, best run so far on ET streets is 13.02@106 with a 1.87 60ft.