Fuel Guage
 

Some 2005-2006 Mustang vehicles may have the MIL on with DTC P0463, or erratic fuel gauge concern. This could be from sulfur contamination present in the fuel. Sulfur will cause an open or high resistance on the fuel sender card leading to an erratic fuel gauge reading or setting a P0463.

I just filled up my gas tank in my 1990 Ford Mustang GT 5.0 and my gas guage doesnt show it to be full. It shows it on the line above the half mark. Anybody have any ideas? Cause it really sucks cuz now i'll be putting so much more gas in my car.