Spectra fuel sending unit

There are two problems with non original senders.
1...the resistor is originally tapered, not only physically but resistance. That is actually an electronic term. The sender resistor is not "linear" taper
2..the aftermarket senders (some at least) don't have the proper full movement of the float due to a shorter than original float arm

However if you get one that is "not terrible off" you can correct the reading with a device called a "metermatch"

MeterMatch Fuel Gauge Interface Module