I'm interested too. My gauge just wandered between empty and 3/4 full, with a lot of time at empty. It has never behaved this way before. I figure I've got several gallons in the tank, based on odometer at last fillup and typical mpg for the driving I've been doing since the fillup.
Note that while the sender "measures" by changing resistance, there is voltage on the wire. And it makes sense that if the voltage is lower than it should be, the reading won't be right. The gauge is operated by magnetism created by the signal, and the strength of the signal depends on voltage as well as resistance.
Umm, right? Right?