Fuel Guage messed up
 

I have a 1996 GMC 2500 Sierra with the 6.5 Turbo Diesel. It has the 35 gallon fuel tank. Since i have had the truck my fuel gauge has not read properly. When it gets just to the "F" i have already used 8.5 gallons of diesel. When it gets to 1/4 my engine starts sucking air. Is there an adjustment in the float arm or do i need to replace the whole tank sending unit? Also i looked on SS Diesel and they sell a In-Tank Fuel Strainer / Sock for $28.00 where does it mount? I dont know much about fuel tanks and i only want to drop it once and fix everything.

my gasser 3500 does the same thing. its always read above full for about 200 miles or so when it gets filled up. Ive seen it on a few other trucks too. I'd be interested to see if a new sending unit would fix that. i just never bothered with it