Gas Gauge Problems

[SnowNeck]

I've just read through a few pages of threads on gas gauges and nothing really fit. Recently I ran my trucks gas tank down to 1/4 of the final quarter put like $20 in seemed fine, drove around for a few day noticed the needle hasnt moved. put $25 in and it seemed to look alright again... Then realized that the gas gauge wasnt only correct when i first started the truck up. I decided to fill it up and see what would happen. It went like a little bit past F which is also normal for me when I fill it. Now it went down to the Full line and stayed there for 2 days found out that its currently 6.5 gallons off so far. Its a 1985 GMC High Sierra stepside with a 20-22 gallon tank I believe. The gas tank has been relocated to the rear of the truck where the spare tire would be. Oh Also with in the past few weeks i changed all my dash bulbs to LEDS so i had to take the gauge off... Just looking for suggestions.

Thanks - Jake