Aftermarket tank & Sending Unit not working correctly with OEM Gauge

[buckskins4ever]

I need help folks. I purchased the Aftermarket Frame tank from LMC Truck and it came with a Sending Unit......I just got it installed yesterday and all seems to work great for except one thing.......................I have at most 8 gallons of fuel in the tank, and the gauge shot past the FULL mark and kept going for another 2 inches. If you can imagine the gauge as a clock face, the fuel needle is pointed at about 4pm when it should have stopped about the 12 since it was only about half full.

Has anyone installed this tank and come across this issue as well??