fuel gauge and/or sending unit troubles

[chevyfan420]

a couple days ago i switched my 79 back to gas from propane. i hooked the single wire for my gauge up to the wire coming from the sending unit and i bolted the ground wire from the sending unit to the frame. i poured a 5 gallon can in, started it up and my gauge read 1/4 tank. I thought ok, alright it works. later that day i put $25 in and my gauge never moved at all. the truck sat for a day. the next time i got in the gauge read 1/2 tank. i drove a bit and found a cheap station so i topped my tank up and it still reads 1/2 tank. later that day it reads a little under 1/2. does anyone have any ideas why its acting so screwy? does this sound like its my gauge or my sending unit? any help is greatly appreciated.

ps: anyone near Ontario want to buy a complete propane setup? tank to carb, it fits on a regular 4brl spread-bore intake.