Patrick,

The electrical value of the sending unit and gauge must match.

I'm going to guess that a '63 has a common 90 ohm sending unit and I'll bet your gauge might too. See if the install instructions for your fuel gauge specify you install one of their senders too.

If everything is matched properly you can bend the float arm on the sending unit itself so the FULL-EMPTY readings are more accurate.

If this error came about after your new gauge was installed it might be a mis-match with the sending unit.

In my area, $20 dollars worth of gas would hardly move the needle at all...my old Silverado takes $88 clams to fill with a factory tank! Let us know how it turns out.

Stuart