fuel gauge problem

[69camarokid]

wasnt really sure where to post this so fell free to move it if necessary.

I have an aftermarket sunpro fuel gauge in my 69 camaro.
For some reason the needle, when the tank is full, will sit at 3/4 full.
And as the tank empties the needle will move towards the F (full mark). When the needle is at the F, the tank is empty. Its really weird.... and annoying. And causes me to run out of gas all the time.

Ive changed out the fuel sender and nearly every part on my fuel system is brand new. But i had this problem befre ai put all those new parts on it. When i got the car with its original gauges it read the gas incorrectly, not in the fassion it does now, but still it was way off. Any ideas on how to fix this would be great, thanks.