Fuel gage : This is dumb

[Beer can]

In my '75 GMC the fuel gage does not work . With the key off it reads 3/4 full no matter how much fuel is in there , Key on the needle pegs way past full. I'm pretty sure this is a common problem because this is not the first square I've owned that had this problem. Usually I just fill up ever couple days ,but after running out of gas as I pulled into work Monday I thought this is dumb. Any ideas on we're I should start and what to look for ? Thanks