How far off is your original Tachometer ?? when I rev up to 6000rpm (dashboard tach) my megasquirt datalog shows 5400rpm also my shift light turns on, set on 5400rpm..600rpm off !! is that normal ?
There is an adjustment on the tach module that allows you to adjust for correct reading for a singe narrow range about 400 RPM wide.. Anything outside that range will be high or low. The upper RPM range is finicky - if it is spot on, it is off at idle and cruse range from 200-400 RPM. The factory probably set the cruse RPM to be the most accurate on the tach