For anyone buying these cheap tachometers and have the same issue I had, I found a solution. I see people all over the internet and in the Amazon reviews saying that these tachometers don't work, and that they are jumping around.
First of all, when you get your tachometer make sure the number of cylinders is set correctly and that the wiring is all connected. If it still jumps around, the signal from the ignition coil could be 'overloading' the tach in a sense, and a resistor is needed.
The resistor needs to be connected in line with the signal wire (the one connecting to the negative terminal of the ignition coil, in my case the green wire). I first tried a 10k resister and had to effect. Then I tried 220k and it jumped around, but now in the lower RPMs. This gave me hope. Finally, I connected a 470K resistor and the tach showed idle at 900 rpm (which was correct, engine had not fully warmed up, and I double checked with an automotive DMM that measured RPM through indictive coil over spark wire).
I have no idea why this tach doesn't work from factory, or why I can't seem to find answers for it online. Maybe this is just for my car and for others this would have no effect. But for anyone who bought one of these tachometers and it is jumping around, try this before you get rid of it. Lots on Amazon and eBay are just rebranded of the exact same thing so there's hope that this will work on a lot of other tachometer listings.