If it's like the later-year ('60s and '70s) fuel gauges, what the needle points at when the key is off doesn't mean anything as there is no power flowing through the gauge circuit. A needle that points at F with the key on and stays there is the classic symptom of an open circuit. The ground wire coming off the fuel tank could be missing or poorly connected, there could be a break in the wire between the sending unit at the rear of the car the gauge at the front, or there could be a problem with the sending unit itself. The first two are easy to check, and if they both check out ok, then it's time to suspect the sending unit.


The gauge is just an ohmmeter and the sending a unit a rheostat. When the tank is empty, the sending unit is at zero ohms, and that points to E on the gauge. When the tank is full, the sending unit is at 90 ohms, and that points to F. With an open circuit, which is infinite resistance and which is much larger than 90, the gauge will often move to well past the F, depending on the particular model and year.

 

 

I believe your fuel sending unit is 0-30 ohms.

 

Might be 30 ohms if it is like a 55. I'm not sure when they changed to 90 ohms but if you get 30 ohms it could be correct. Does anyone know when they changed for sure for my own curiosity .Also a bad grounds is a common issue on 55's. The clamp on the gas line that holds the ground wire corrodes and often will give a bad reading. This may not be a issue on a 58 but would be worth a look at.
.... Tedd

PS Eric you beat me to it again. I just type slow...

 

I thought it was in the 60's for all GM.

 

Team Chevelle stated 64 is 30 ohm 65 is 90 ohm, most likely my new sending unit reads full when the key is on....Say's you have to change the gauge in the dash to 65?? Crap....

 

I checked my receipt with Summit and mine is part number FG88D fuel sending unit 90 ohm and under applications says it works on 64 A body's? My grounds are fine, may check up under the dash in the AM. Bottom line is, I hope the guy on Team Chevelle is wrong...

 

AHH, AHH Team Chevelle was correct, the number ohms needs to be the same between the gauge in the dash and the sending unit in the gas tank-end of sentence. Up to 64 it was 30 ohms. So that said, the best way for me is I'm going to order a 65 on up Auto Meter 90 ohm gauge to work with the 90 ohm sending unit in the gas tank. To make life easier I will pull the Auto Meter volt meter out of the 3 gauge panel that's under the dash and replace it. Then have water temp, oil pressure and then fuel....Never ends does it??