Gas guage doesn't work. When its full it reads full. But when its empty it reads 3/4 full. Could it be the sending unit or the gauge itself??????

 

bumppp

 

If u wanted to skip all the diag BS, I'd say your fuel sending unit is not sending the correct information.
But it might even be something as easy as a loose harness or some crazy ****,
either way best of luck.
Sorry I couldn't completely help.