I think you need to test the gauge's response to at least two more resistances.
The test you did here suggests the gage and connecting circuit are working correctly, but the sending unit is not.
i did that before ordering a new sender, I then tested the new sender with an ohm meter. Engine off (70 OHM) then running(20-25 ohms) I do have 45 PSI at idle 55 PSI at 200 rpm checked with a snap on guage. I still have to do a resistance check, but I am in a natural mental process of elimination, the other guages work, and the dash PC board hasn't been replaced, still the 50 year old unit. The PC bords are available the guage and dash not so much people are selling junk on e bay with bent indiactoirs for $100 bucks LOL!
Why do I say that?
Ask yourself at 45 psi what should the sender resistance be?
