since i got this truck in june 2011 i havent had a fuel gauge...i dropped the tank and found out the brass float on the sending unit was cracked and full of gas. well after gettin every drop out my father (r.i.p.) brazed the cracks and got it sealed up and i put it back in...i had a fuel gauge for about 3 days and then it went back to empty and stayed there again...well last night i dropped the tank again and put a new sending unit in...got it all hooked up and tank back in and BAM!! the gauge shot to full with an empty tank...so what causes the gauge to show way past full no matter what?

 

Ck the ground on the frame..... it should be right there near where the sending unit plug is. And ck the wire harness going down the inside of the dvrs side frame rail and then where it goes up to the cab. Sometimes the emergency brake cable cover will chaff on it and cause a broke wire.

Did you get the sending unit in correctly, maybe a dumb ?, but could it be up side down. Been a day or two since I have dropped a tank and changed a sending unit.

I have seen a trick where before the new sending unit it installed it is connected and held in place like if should be and key on and move the float arm to see it its working or not on the gauge,

 

It sounds like either the new sending unit is bad or the wire going to the sender is grounded somewhere.
Unplug the sending unit and see what the gauge does. If it still shoots to full, the wire is grounded somewhere between the gauge and sender. If the gauge stays on empty then it sounds like a bad sending unit.

EDIT: Are you using the factory plug, or have you converted to crimped on ring terminals?
The reason I ask is the factory plug only goes on one way but ring terminals could have the connections reversed. I'm not sure it would direct ground like that but it could be a possibility.
Here's what the factory plug looks like incase you have ring terminals.

 

i have the factory plug on my truck...ill have to check that ground wire...i wish i hadnt have had my head up my rear i woulda checked it all before i put it back together...my goofy self gettin in a hurry tryin to race daylight...

 

Seems that in the back of my fuzzy brain there is a question about the matching of resistance of your sender to your meter.
If the new sender you bought has a different working ohm range than the meter/gauge in your truck it will not read correctly.
So after you check the ground, check your senders ohm range compared to the gauge. Hopefully it's stated somewhere on it or in paperwork.
I believe Ford senders operated in the 10 to 50 ohm range. 10 being full and 50 empty.
Just thinkin' out loud here, not fully sure of what you have going on.

 

Been there, done that.......many times

 

I'm just not gonna mess with it right now...ill be pullin the bed soon to put the repo unit in. I gotta move the tank anyways. I wish id thought about this before