MAP/Fuel Pressure Gauge equivalency

[Tcraft938]

Probably a dumb question, but after reading the thread and appreciating Carusoam's comments, I'm wondering if maybe my MP gauge does not need overhaul but perhaps a line issue?  Description of the problem.... when sitting on the ground without engine running, the altimeter setting is 30.41" but the MP is reading around 27".  In flight at 9,500' density altitude, per the POH, WOT and 2400 rpm should yield 20.2" for 69% power, but the MP is showing a max of 18".  Fuel Flow and TAS however, at this setting are in agreement with the POH.  On landing for example, when I pull power back to the point the MP needle is pointing straight down at 10" I have more throttle travel and the engine reduces power and RPM further.   So it seems no matter what my gauge is reading about 2-2.5" lower than what is really going on.   

The previous owner placed in the POH a nice cheat sheet on power settings to enter the pattern, fly ILS, etc.  When I set to these setting the plane is always substantially faster than he states (such as gear extension).  If I subtract about 2" from those settings, speed is right on.  So does this sound like a gauge issue, or could there be simpler things like the line to check, before we pull the MP/FP gauge out and send it out for Overhaul.  btw 1963 M20D converted to C    Thanks

[David Lloyd]

Tcraft, sitting on the ground without the engine running, the altimeter setting is 30.41, the MP should be reading about 29.7" if at Brockport, about 665' MSL.   That error could be at one end of the range or across the whole range.  From what you wrote, sounds like the entire range.  Live with it and compensate or have it overhauled.