Fuel Gauge Question

dibbons said

This seems to be a universal inaccuracy--my factory gauge/factory sender/factory pickup work together to read 3/4 when tank is very full. We all have been cursed by the same hex.

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Please don't assume the gas gauge didn't read correctly when new. Although your fault could be as simple as a small amount of fuel inside the float ( we hope its that simple ), there are several other possibilities too. A failing limiter can cause all the thermal instruments to read below accurate. And compromised chassis ground path of that limiter does the same.
I've opened and renewed a few hundred of these instruments so I've seen the affections they get with old age. This is most typical of the fuel gauges simply because of their duty. They spend a lot more of their life at the high end of their range.
So... Agreeing that the aftermarket senders aren't proper, at 9 to 10 ohms the gauge should still go to full even if its inaccurate at the lower levels.
Measure your sender resistance at full tank. Approx' 14 ohms would be approx' 3/4 hash mark on the gauge screen.