patrickm,
The 54/55 gauge is a little different from earlier gauges:
here are a few photos but the principles-of-operation are the same.
As stated above above, bad grounds and no grounds (resulting in a needle "pegged" to Full) are common problems. And, unwanted grounds can also lead to problems.
Look at the two photos at the link:
1. In the uninstalled (off) gauge photo, look at the ground metal between the posts and note a circular metal?-thing in the middle, and note that neither post is connected to the ground.
The gauge-housing is grounded and it is usually a good ground; however, make sure the gauge attachment-nuts and where they touch the dash are good/clean-metal.
NOTE: when the gauge is installed into the gauge housing, neither the fuel-gauge posts nor that circular thing between the posts touches the housing.
2. Look at the installed gauge and notice two things: 1. the red label indicating "sender connection" is on the sender post (and must not be connected incorrectly); and, 2. there is a piece of cardboard that goes on each post and in-between the posts - this is to assure that you do not accidentally touch the wrong lead to the wrong post.
The most important ground for the gauge/needle circuit is between the sender "cap" and ground. For diagnosis/testing purposes, I sometimes run a "ground-confirmation" lead from a sender cap nut/screw to the frame or some other good ground (to make sure that a bad reading is not due to a bad sender ground).
Good luck,
