The tachometer can be calibrated pretty easily; there is a potentiometer in it that allows you to calibrate it to be correct at one specific RPM, which should make it more accurate through the range. There used to be some detailed instructions online, but I can’t find them anymore. To the best of my recollection, all you have to do it open it up (give it a good cleaning while it is apart), find the potentiometer, then drill a small hole though the back of the housing opposite the pot. Put the tach back together and re-install it. Hook up a separate dwell/tach so that you have an accurate reading. Then, it’s as simple as running the engine at the speed you want to set it for (I set mine for 3000 RPMs) and using a small screwdriver to adjust the pot until the car tach matches the dwell/tach reading. As I recall, I had to do the adjusting on my back under the dash, so it helps to have a second person to hold the accelerator to the desired speed and read the tachs. You could also temporarily rig a longer harness so that you can hold the tach while you work, but I just asked my wife to help. I put a piece of tape over the hole when I was done to keep dust out.
At least, this is how I remember it working. I am sure someone will correct me if I am wrong.