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Just want to relate my experience with the fuel level senders, for anyone else having this problem, and because this thread was the closest thing I could find from days of googling this.
Problem: fuel gauge doesn't fall below 1/2 - 1/3. Started abruptly, not going below 1/4 for a couple months, then got worse. 2004 RX8 MT, 260000 miles.
Cause: both fuel level senders' resistor cards became less accurate toward the bottom (tank empty) ends.
This really confounded me for a while because my brain really hates it when it seems like two distinctly different parts fail in exactly the same way at exactly the same time -- especially after so many years/miles. But it's true. I removed both senders and tested on the bench. They both returned 10Ω on the high (tank full) side, and 263Ω on the low (tank empty) side. Identical. It wasn't until I actually consulted the shop manual that I realized the low side is supposed to return ~325Ω.
I used a little pick tool and tiny wire brush to clean off the varnish from the contact strips, and sure enough, both senders low end results came back into spec. I have two new senders on order, but for now, these old ones are working fine again.
Moral of the story: there is a remote chance that, if your gauge isn't falling below 1/2, it could be both fuel senders are a little faulty.
On a side note, there is disappointingly little info about this coming back from my internet searching. I never found anyone discussing why this varnish thing matters or how to deal with it aside from replacing the part. I don't know much about electronics, but my guess is that the varnish is a pretty good conductor, therefore the more of it there is, the less the electricity is forced through the resisting bits, therefore the less resistance Ω gets added to the circuit?
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