gas gauge reading when full but goes down quickly

I replaced the tank sender in the 69 Barracuda a few years back and it's never read correctly. I do have a solid state 5v IVR and it's working just fine. Temperature gauge reads correctly too. Anyway, what happens is the gauge reads full when I fill the tank but goes down quickly to near zero when I still have 1/2 a tank or so.

So the question is what is the best way to get the calibration right or nearly so? Do I just have the wrong sender? I'm tired of guessing who much gas is left in the tank.