1965 dart fuel gauge doesn't read correctly

easyrider said

I have a jumper wire from the sending unit to the body.

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Are you sure its a good connection? Not just the factory strap, they tend to loose connection. Once your sure of the ground, check the sending unit wire for good connection. Pull it off and make sure its snug on the post. Slide it on and off a few times to clean the connection.
Is this an original sending unit? Sometimes you have to pull them and clean off the winding. Also inspect the wiper, sometimes they can wear a hole right in them. If the wiper is ok check the tension. You can carefully bend the tab to get some more tension of the wiper to winding.