Fuel gauge only going to 3/4 when grounded?

fshd4it said

So, got a couple things figured out today. Swapped in a spare IVR, now all 3 gauges go full tilt when grounded. Fuel tank sender shows 43 ohms, but I don't know how much gas is in it, so I'm not sure if that's accurate. Also, with everything hooked back up, the gauge shows about 1/8 tank. I'll fill it up tomorrow, see what it says then. And the oil psi gauge going full when disconnected? The warning lamp (for a dash without a gauge) was in there, back feeding 12v to the signal side.

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I've never seen a panel that had both warning lamp and gauge for oil. Warning lamp does operate on 12 volts. Only way that could work is with two separate senders and wires gto them. Anyway...
43 ohms is approx' 1/8th gauge scale. Chryslers service tester uses only 3 points of the scale, 73 ohms at empty hash mark, 23 ohms at 1/2 hash mark, and 10 at full hash mark. The quarter hash mark is approx' 34 ohms, 3/4 hash mark is approx' 14 ohms.
Those adjustments on the back of the gauge are for the factories calibration only. The end user should never need to mess with them.