I have a 1988 ford f 250 with twin fuel tanks. I believe they are both 20 gallon tanks. If I put 10 gallons in the truck it reads way way past full, after I use about 7 gallons it will go to almost empty and fluctuate around, as I use more fuel it will eventually go to empty. Both tanks act the same way. If I remember correctly in the 1970’s ford had a voltage limiter that controlled the fuel gauges. Was this still used in the late 1980’s and if so would it possibly act like this?
Ground across from the body to the sending unit. Does it work better ? 
The bad ground at the sender will cause the meter to read higher, and go eratic as the ground’s capability varies.
There are other possibilities.
There’s an anti-slosh device in the dashboard part ?
or there’s a way to test out the sender to see if its making an sense down at the sender. It may be a totally stuffed sender.
When he says “voltage limiter” he must mean “voltage divider”.
The gauge is a volt meter.
The sender is a resister … So how does the volt meter measure a resistor ?
The varying voltage comes from the use of two resistors in series - the “voltage divider”.
One of those resistors is the sender and the voltage in the middle varies with current, the current is varying with the amount of fuel in the tank due to the sender varying resistance.
No, I am taling about something called a voltage limiter unique to the older ford products. I actually repaired trucks for a living and repaired fuel senders and gauges almost daily. I haven’t started checking my own yet as I was going to try and find out if it had one of these voltage limiters before I started checking it.
It has been over 30 years since I have worked on the fords but it seems the limiter was used for the fuel guages and possibly the temperature gage. I will try and research it online a bit.
I remember now, the gauges were 6 volt for the fuel and temperature, so a limiter or regulator in the cluster dropped it from 12 to 6V.
I’ve got an ‘88 also and as I recall (been drinkin’ beer tonight), the tanks are 18 front and 16 rear, which seem puny with the thirsty 460 up-front.
Anyway, my guages do the same thing. Fill it up, they peg out to the right, then drop suddenly like a rock to about a 1/4 tank, slowly go well past empty.
I only rely on my odometer to guage when I need gas with this beast.
Also, the (useless) electric tachometer goes counter-clockwise to peg out when you turn on the key. Another ground issue?
Where the hell is Rick?
Well, there’s also this in general…
he’s adding to the mother of all disclaimers.
This thread might be helpful. It’s a Ford Truck forum.
The diagram in my repair info is a little hard to read (blurry), but it seems pretty clear that there’s no voltage limiter. The five gauges are fed directly from the same power source. If the gauge acts the same whether switched to and filling the front tank or switched to and filling the rear tank, I agree that checking for a faulty ground is a good place to start.