This is my first post so here goes. I have a great restored 58 Olds that that has a fuel gauge that has progressively been failing for years. It now barely registers fuel. I would like to know what is the most common cause for this car and also what the easiest approach would be to identify the culprit. The service manual assumes I have an original set of diagnostic aids etc.
I had one of these back in 1961 just like it and amazed many Impalas and Fords at the dragstrip.
Help!

 

Welcome to the site, post some pictures when you get a chance. First you need to disconnect the wire that runs to the sending unit from the gauge at the tank. With the key on and the wire removed the gauge should go past full. If you ground this wire the gauge will read empty. If this happens the problem is related to the sender.

 

Before you drop the tank check the ground wire from the sender that(at least in the 55-56 cars) is attached to the gas line with a clamp. This clamp is the root of many gas gauge problems as it corrodes and provides a poor ground . Also if you have made some kind of repair on the gas line in the past and have used a rubber hose to put the gas line back together this also will stop the ground and the connection won't be complete .. Tedd