Either need a new gauge, or a new fuel sending unit for the fuel pump in the gas tank.
I have the same prob, I narrowd it down to gas gage
the first thing i would do id take the gas tanke out and replace the sending unit. i had that problem when i bought my 1988 gt
I replaced my gauge cluster with a 140 m.p.h. speedo. Both clusters gas gauges worked but were quirky. The new doesn't always go all the way to the top when full. First thing I would do is take the gauge cluster out. Take the face plate off and see if it is hung on something. Then hook it back up and see if it works. If you want to buy a new cluster I have one I'll sell you. 85 m.p.h. 59k. It came out of my 88 LX convertible.
Have you tried unplugging the the gas line and reseting the guages? The same thing happened to my '99 GT and my friend's '88 notchback. Inside the gas tank there is a small float which sends the readings to the gas guage. Most likely the float fell down into the tank or got clogged with something(You'd be amazed what people find in old car's tanks). I would suggest replacing the entire float mechanism, which involves dropping the gas tank out of the floor. Most local mechanics can do it. Choose wisely.
Looking for a Used Mustang in your area?
CarGurus has 45,840 nationwide Mustang listings starting at $1,999.
Search Ford Mustang Questions
Ford Mustang Experts
Related Models For Sale
Used Cars For Sale