Gas Gauge Goes Way Bellow Empty
 

I replaced my right side fuel gauge in my 69 with a tach and installed an aftermarket center fuel gauge. Old, original gauge works fine. The new center gauge works fine with key is on but when I turn key off center gauge goes way bellow empty (8 o:clock if it was a clock), original gauge does not do that. Thought it was a bad gauge so I got another new center gauge and it does the same thing. Rechecked original gauge and works fine still. Pulled sending unit and checked and it checks out. The place I purchased the gauge from says the gauge should not do this. I believe it is a problem with the gauge design and the selling company is wrong, I can't believe GM gauges had this situation going way under "E"
Anyone out there seen this issue? Any cures?