Why is my Tach "off"?

[KNS]

My tachometer reads low. Example: 70 mph in fifth gear will only show 2500 rpm. It reads low through the entire range and I can't figure out why. The tach has been checked by North Hollywood Speedometer and it's fine. The CDI box has been swapped with a known good one and the problem is the same. Any ideas? If I hooked up the wires on the back of the tach incorrectly (I don't think I did) would everything still work as well as having the low reading? Any help much appreciated......