I have a '98 Scab Lariat, with the mechanical odo. I've just installed an Edge Evolution programmer, and before using one of the canned tunes, I've noticed that on this, and an Actron code reader, that the tachometer reads 350-500 RPM higher than what is being read off the OBD-II port. MPH on the gauge, and Evo display are either dead on, or within a mile or 2 per hour of each other. Why such a discrepancy, and is there a way to fix?

Worst case, I can keep the Edge programmer installed on the dash, and remove it like the faceplate of my aftermarket JVC head unit.

 

You can pull the needle off the tach and reposition it. I just got done doing that.