I went through this witn a SW temperature gauge. I found that 100 ohm corresponded to 155 F or so. With the gauge contacts reversed (12V to A, resistor to B) the needle pegged low. When connected properly, it read properly.
Try two tests with the lead at the fuel sender. Disconnect it (high resistance) and see which way the gauge reads, then ground the lead (low resistance) and verify that it pegs the other way. Finally, check that this corresponds to your level sender. If it doesn't, I'd have to guess you have the wrong sender.