are you still 6volt on your 55. the prev. coments are a good start.my, 1949 pickup has 12volt with a votage reducer from 1959 ford car and sending unit late 60s truck, it works fine. sending units are a issue as the stock tank wont take later sender. 6 or 12 will work as long as u use the voltage reducer to the hot side of the gauge.if you are converted to 12volt.

Does the gauge still read full when you turn the ignition key off? If so, the gauge is stuck there. If not, disconnect the sender wire at the sender and make sure the end is not touching anything. If the gauge does not move when you turn the key on, then your sender is either grounded or faulty. If it does move back to full, then your wire is grounded somewhere between the sender and gauge. Hope this helps you find the problem.

What year and model?

55 Ford Crown Victoria Steel Top

Probably been discussed here someplace - can't find it.  My gas gauge always reads full.  Can someone help?  Thanks!