[Wgesnerjr]

One thing to keep in mind that the accuracy of the stock gauges was not that precise. However if it is reading high after the tach install, I would think that has more to do with it. Since your testing of the battery and alternator showed no issue, I would start with checking the power at the gauge and see if it measures the same as the alternator/battery. if so, I would say the gauge is not accurate. Might also check the grounds at the dash to make sure they are clean. Poor grounds have been known to do weird things.

hope this helps.