when I have little less then half tank of gas gauge drops to E why ?

Suspect in-tank fuel level sensor .. Can be confirmed by scan test with professional quality scan tool... Scanner has capability of testing fuel gauge and can monitor fuel level sensor on live data screen... IF scanner readings concurs with fuel gauge, sensor is culprit .. IF scanner readings differ from fuel gauge, fuel guage is culprit... Most independent repair shops will be equipped to do the 'required' testing... Keep in mind that some fuel gauge deflection is normal while driving but it shouldn't drop that much from just under 1/2 tank... **Does the fuel gauge return to same reading when vehicle is parked OR does it just remain on empty - until next re-fueling?**

This is extremely common, and is the result of sulfur buildup on the sending unit in the gas tank. More than the C5 is effected, too. Everyone has their own brand, but get some fuel system cleaner that you like, and let it sit a couple days in a full tank of gas. If it doesn't fix it, you'll have to go in through the wheel well and swap the sending unit. Of course if you have to replace it, test it first, but 99% it's bad.