Gas gauge help

[RRR18]

I just replaced my underbed tank with a replacement due to leak. My problem I think is my gauge. So I'm leaving it to the experts here to point me in right direction. New tank included new pickup and sending unit assembly already installed in tank. Just swapped the tank basically. I know that my ground for the tank is good and the wiring is good. With the old tank - even when it was full the gauge never read past 3/4 full and would still be running under E. Now with new tank - I put exactly 5 gallons in to get everything running this past weekend and the gauge moved slightly but not past E. So I did same thing today and went and got exactly 5 more gallons and it moved the needle to 1/8 of a tank. In my mind I put 10 gallons in a 21 gallon underbed conversion tank so am I stupid for thinkin it should be at 1/2 tank on the gauge right now??? Or is my gauge kinda bad? I thought either the gauges worked or they didn't. It's a '67 but I don't think it's a '67 cluster. Could be anywhere between 67-72 cluster. Any suggestions???