Installed new coil, now the tach acts funny

if you have exhausted all possibilities

try undoing cleaning up and redoing the ground strap between motor and chassis
one sure fire way to make all of your instruments go weird is to run the car with no earth strap or one so corroded it doesn't connect anymore

motor then earths through throttle cable, speedo cable into the dash if the cable tube it uses isn't nylon lined .
through the coolant and radiator

none of the devices that use the ground at the engine can see a "true" ground. so they can not measure anything against true ground, their ground varies with every spark.
and depending on if your ignition switches the coil off and on by grounding or by pushing the negative side to the same 13+V as the positive the ignition module may know what ture ground is but that's worthless if the engine isn't at true ground, really depends what the ignition is bolted to.

temperature reads wrong
oil pressure reads wrong
tach needle just buzzes at some random level
you appear to have either way more or way less (depending on year) fuel
car runs kinda ok but is hard to start.
often easier to start if you press the throttle which puts tension in the throttle cable making for a better ground connection between it an the metal ferrules on the throttle cable clamp and the bulkhead clip.

probably wrong but you never know...

Dave