The OEM ones needed "tweaking" sometimes.
I remember working at the dealer in the late 60's, early 70's and there was a chart that showed the resistance from full to empty. If you wanted it to be spot on you measured the resistance of the sending unit full sweep from full to empty. Usually 10-73 ohms. You then picked the resistance for 1/2 a tank and we had a tool that you could set the resistance with (nicknamed the "dial a prayer") . You connect that to the wire connector at the sending unit and check the gauge. If it wasn't exactly on 1/2 you could pull the instrument cluster out and adjust it through the little holes on the back of the gauge using a small screw driver. Didn't do it a lot but occasionally we had to do it to satisfy a customer
I remember working at the dealer in the late 60's, early 70's and there was a chart that showed the resistance from full to empty. If you wanted it to be spot on you measured the resistance of the sending unit full sweep from full to empty. Usually 10-73 ohms. You then picked the resistance for 1/2 a tank and we had a tool that you could set the resistance with (nicknamed the "dial a prayer") . You connect that to the wire connector at the sending unit and check the gauge. If it wasn't exactly on 1/2 you could pull the instrument cluster out and adjust it through the little holes on the back of the gauge using a small screw driver. Didn't do it a lot but occasionally we had to do it to satisfy a customer