Chosen Solution
I purchased an aftermarket tachometer for my 1990 Geo Metro 3cyl because the car did not have one. The tach I bought (Link to Tachometer) is supposed to work for 3 cylinder engines. When I connected it up, set to 3cyl mode, it would jump all around from 5-8k rpm. I set the tach to 6cyl mode to see if it jumped around lower but it still jumped around 5-8k rpm at idle.
