I went through this drama a few years ago, including installing a Runtz that worked very well; until a few years later after the introduction of alcohol in the gas. I thought the Runtz had failed. By this time 12V gas gauges were available for 47-53 trucks so I installed one. The gauge still read low - never over 1/2 tank. So I bought a sending unit and installed it. Gauge never moved at all after that - stayed at 3/4 no matter what. So I took the sender out and examined it closely. I discovered that the hole pattern on the sender caused the float to jam against the side of the tank. So I got out the old sender and looked at it carefully, measured the ohms, yep 30 ohms like its supposed to be. Hmmm, the cork float seems heavier that I would expect. Put the cork in a can of gas and watched it sink toward the bottom.
So I put the old sender back in with the brass float off the new sender in place of the cork one. Gage has read correctly ever since. After the cork float had dried out it floats once again but it'll stay in the parts box with the new mis-drilled sending unit.
