1986 f150 fuel guage problem
 

I have an 86 f150 4x4 with the 4.9 liter straight six engine with dual gas tanks. The fuel gauge always reads empty, except when the tank being used is actually almost empty, then the needle will rise to about a quarter tank or so. Since this happens for both tanks, does this mean that it is not a fuel sending unit problem (I'm assuming there are two sending units), but instead a problem with the gauge itself? Any help would be greatly appreciated, thanks.

i believe there is a unit in each tank...the float or unit is "tired" so its best to replace them