Q: Is the 2.25" size the dimension of the square face or the diameter of the round face of the instrument?
It is the diameter of the round face of the instrument.
Q: what are the outside dimensions of gauge?
The outside dimensions are 2.25" L by 2.25" W.
Q: Does it use a electrical sender or a vacuum hose goes to the instrument?
It uses a vacuum hose.
Q: The diameter of the round face and the height and width of the gage cannot both be 2-1/4 inches.
Would you please verify the dimension on this instrument? Does it fit in a standard 2-1/4 instrument hole?
The diameter of the face is 2-1/4". The height and width of the gauge is 2-13/32". Yes, it fits a standard 2-1/4 instrument hole.