Gas Gauge

[FCShively]

I have been restoring a 1977 C-10 for 4 years. I am putting the dash back together and have encountered a problem. I installed a new gas tank, sending unit/float, wiring and gauge. I wired the gauge straight to the truck as I am using a combination of factory and aftermarket gauges. I ran a switched hot to the gauge, a ground and a wire to the sending unit at the tank. The tank sending unit has a good ground. I put about 8 gallons of fuel in the truck and the gauge is reading full. Any suggestions or anyone know what I have done wrong?

Thanks for any help