they are all over the place... some are dang near dead on, some are several hundred rpm off, some are good at steady cruise, but under acceleration swing slow, so lag behind actual rpm when accelerating... causing you to over rev.
The good thing is, they all seem to be consistent in their own/singular function, so, once you get your shift points dialed in it's easy to repeat runs off the factory tach... regardless of whether the indicated rpm is accurate...
My dash tach stays steady during small rpm swings while idling. The MSPNP 2 and Autometer tach are dead on with any movement. The stock one lags a couple hundred rpm when above 3500.
Update, I pulled the gages last night and the printed circuit seems ok. The circuits to the amp meter are melted, which was new to me. Not really important. Have not had the gages out in four or five years. Pita to remove.
Anyway the tach seems to have died, so I swapped in the Sunpro from the Chevelle and it seems to work. I did take out my Proform shift light, but maybe it and a plain tach would work.
Mostly looking to see if a $400 tach is worth it for occasional bracket racing.