Any suggestions? I have a 1986 F-150 with a 5.0 AOD, the rear tank is not on the truck so I only use the front. The gauge works as far as incremently going down with use but when I fill the truck the gauge reads past full and when it is empty it reads there is still a quarter tank left,

 

Classic sending unit problem I would guess. If you got in the sending unit producing business for these trucks, you would have lots of business.

 

Do your oil and temp gauges read high also? If yes, the Instrument Voltage Regulator (IVR) may be putting out too much voltage. That can be adjusted down. If it's just the fuel gauge reading high, it could be an issue with the the float itself. You caneleiminate the gauge and wiring as an issue by disconnecting the yellow/white wire at the sending unit and grounding it with the key in the run position. It should swing to full with the wire grounded and empty with the ground off.

 

If it is the sender they are available aftermarket for the EFI motors 19gal(60") midship tank as an assembly.

 

if its your IVR here is a good one to use.

Ford Instrument Cluster Voltage Regulator 5v - eBay (item 280572904377 end time Apr-06-11 18:52:57 PDT)

 

I've had that one in my truck for about 4 months....it's every bit as good as advertised.