tach misreading - showing wrong rpm's

is that possible? currently i have a speedhut tach. On a MSD6HEMI ignition box. I am not sure if so but it is possible that the Tach is showing ie 6000 rpm but actually the engine is reving 7000 rpm?
Damn good question....I,Would send MSD (Holley?) an email,first. A,possibility.... You would probably get a quicker response,by posting this,thread in the "New Hemi Swaps" forum. Don't forget: Joey ,administration of For A Bodies Only post: a new,server swap,is coming this weekend....Just a heads up.
Can I put an Autometer tach parallel on the MSD Tach port to check that?