What would cause my fuel gauge to read empty when it is full?
 

2001 Chevy Silverado 3500 1 ton dump
Duel tanks one relay for pumps both tanks have FSU .
What I have done :
I have problem with the second tank pump not pumping for some reason. I know this because ran out of gas at half full on the fuel gauge.AND I went to fill tank and second tank was full to test theory I ran truck for 100 miles and fill primary tank and the second tank would not take any fuel it was full.
I have tested grounds for sensors and for the pumps all are fine.I ran test leads to the second tank for the fuel pump and it ran . The second tank feeds the primary tank through the over fill hose on primary tank.Both pump leads on primary and second tank are at @5.88 volts when key was on accessory but did not change when I cranked it over.This truck only has one relay for fuel pumps according to parts dept at Chevy dealership so I am assuming the pumps run at same time even flow type system.What would be the correct way to test the sensors on each tank? I have have fluke volt and ohm meter just need to know the correct readings for each sensor?

This should help you out.

http://i187.photobucket.com/albums/x2/acnas/tank.gif

You may have to expand pic. Looks like full is 250 ohms and empty 40 ohms