wiring fuel sending unit

[impalaboy9]

Hey guys got a Boyd's AL gas tank. Awesome product strongly recommend it. It came with its own sending unit and I'm trying to wire it up to the new dash I built. I have all new gauges. I wired the fuel gauge up by grounding it and hooking the other side to 12v like the directions said the third wire I assume measures resistance. I ran that wire back to my sending unit but the tank reads full and it only has 5 gallons in it. Is the other prong on the sending unit used to ground out the unit itself? Also, the two prongs on the sending unit aren't marked does it matter which is ground and which goes back to the gauge? Thanks for the help.