I just put a new gas tank and sending unit in my 1970 Olds cutlass Supreme.The gauge read 3/4 before I started and when done it sayed the same?I did hook up the ground wire comming off the tank.The tank I bought has 3 other lines comming off it.I pluged all 3.when I unscrew the gas cap I have air pushing out.Any help would be so nice,thanks!

 

Have you verified the gauge is not just stuck? With the gauge disconnected from the tank at the rear of the car, ground the side coming from the gauge and turn the key on. The gauge should immediately go to E. Remove the wire from ground, and it should immediately go to F or past it. If it does those things, the gauge is fine.

But I'm suspecting that, with a new sending unit, if properly wired, and the gauge has never moved from 3/4, you have a stuck needle or an electrical problem right at the gauge that replacing the sending unit did not address.

 

Did you install a vented or non-vented cap?

Have you checked the voltage supply to your gauge and the connection between the gauge and the sender?
There are a number of threads here where fuel gauge troubleshooting is discussed in detail, such as here, here, and here.

- Eric