If you are using the factory tach you probably know it's not very accurate, and over the years probably has only gotten worse (or has crapped out altogether). Greg at Cajun Tach Shop https://www.cajuntachshop.com/ was able to take my original tach and create a new replacement circuit board which solves all those issues. He doesn't have them listed on the web site yet so you'll need to call or email directly. I'm not sure what he'll be charging for these yet but his replacement boards for other GM cars usually run less than $60 which is a bargain. I've been using one of his boards in my '84 Camaro for a while and have been super happy with it.